Long Barrel, or Short? The Effectiveness Trade-Off Between 14.5″ and 20″ Barrels

PEO_M4_Carbine_RAS_M68_CCO

In my comments section recently, I was asked to shed some light on the velocity penalty created by moving from 20″ long rifle barrels as in the M16 to 14.5″ long carbine barrels as in the M4 Carbine. Our readers’ wish is my command, and so here we are.

In the very first edition of the Modern Intermediate Calibers series, I talked about the 5.56x45mm and its performance from both 14.5″ and 20″ barrels. However, the graphs produced for that post lack the clarity we’d like to discuss this issue in detail, partly because they are a little crowded, and partly because I hadn’t quite balanced them for clarity and size yet. So, to begin answering this question, let’s take another look at that data, only this time we’re going to clear out the other loads and just focus on M855 from 14.5″ and 20″ barrels:

buhdOUN

Looking at just the velocity curves for a moment, how do we interpret this data? Well, one of the primary criticisms of the M4 focuses on the fragmentation threshold of the Carbine versus its longer, older brother. Critics say that while the M16 has an adequate fragmentation threshold for normal combat, the M4 – due to its lower muzzle velocity – has a much shorter fragmentation threshold.

However, this talk to fragmentation thresholds is too simplistic, at least when talking about M855 (M855A1, due to its more consistent fragmentation properties, is a better candidate for this metric). Still, fragmentation thresholds are really a subject for another day, so for now let’s take it for granted, and compare the two curves. Before we continue, let’s lay out our assumptions:

1. M855 will produce 2,920 ft/s from the 14.5″ barrel of an M4 Carbine.*

2. M855 will produces 3,150 ft/s from the 20″ barrel of an M16A2 Rifle.*

3. Out to the distances we care about, the velocity curve given by the 0.151 G7 BC figure accurately represents the real-world velocity curve of M855, under standard conditions.

4. For convenience, we will assume a fragmentation “threshold” of 2,500 ft/s, even though this subject deserves a much more in-depth treatment, which we will reserve for another day.

Alright, so where does the data get us?

From the data, the M4 Carbine with its 14.5″ barrel and 2,920 ft/s muzzle velocity meetsย our velocity threshold at 125 meters, while the M16A2 with its 20″ barrel and 3,150 ft/s muzzle velocity meets the threshold at 190 meters.

In other words, the difference in velocity between the two rifles is enough to create a “gap” in performance of 65 meters, or put differently, the M16A2 has by this model a 52% fragmentation range advantage versus the M4.

These numbers are nice, but immediately we should understand that this model has some serious limitations due to its four assumptions. We’ll worry about assumption 3 another day, but let’s take a look at assumptions 1 and 2 for a moment. Do these velocity figures represent the actual muzzle velocities produced by service rifles and carbines, and if these figures are changed, how does that affect the end result? I would characterize these figures as being “nominal” for both weapons, which means that I would expect a brand new rifle of either type firing in-spec ammunition made by an American manufacturer to produce velocities close to these figures. Or, put another way, these are “high” figures that represent the maximum muzzle velocity produced by either type of rifle assuming no errors in quality control and a temperate environment.

What if we don’t have these things? What if our rifles are worn out, or our ammunition was “cooler” because it came from a manufacturer that didn’t load so hot as IWI or Winchester, or what if it’s just damn cold out? Well, we have a good idea how these things affect the weapon, for two reasons.ย First, because barrel life in the context of ammunition testing is defined by a velocity drop of 200 ft/s or greater, over the course of usually 10,000 or 15,000 rounds. Second, we have a very good test from Small Arms Defense Journal written by Dr. Philip H. Dater & Jason Wong, which chilled the ammunition for consistency reasons before testing commenced. From a 14″ barrel, their chilled M855 ammunition gave them muzzle velocities about 200 feet per second below our nominal M4 muzzle velocity above.

Therefore, it stands to reason that some weapons in service will – due to any one or combination of these factors of cold, barrel wear, and ammunition “hotness” – produce muzzle velocities as low as 200 ft/s below the numbers quoted above. This makes our next step pretty clear:

sPeGbzB

In this situation, the difference becomes more pronounced. The M16A2 with a compromised muzzle velocity gives us just a shade more fragmentation range than the fresh M4 under ideal conditions – 133 meters – while the M4 with a compromised muzzle velocity gives a very disappointing fragmentation threshold of 66 meters. This means that in these less than ideal conditions, the M16A2 has a 67 meter – or 101% – advantage versus the M4.

Before I leave you there, let’s briefly challenge assumption 4 by changing focus from M855 to the new M855A1 round. We’ve already noted that the entire concept of a “fragmentation threshold” is overly simplistic and really deserves its own post, but one wrench we can throw into the gears right now is that we know Enhanced Performance Rounds like M855A1 seem to have much lower fragmentation thresholds than their predecessors. We have seen that M80A1, M855A1’s 7.62mm counterpart, can fragment very violently at velocities as low as 1,930 ft/s. Therefore, our “2,500 ft/s” value is likely far too high for these rounds, and not at all representative.

Fortunately, M855A1 has a close enough ballistic value to M855 over the distances we are considering that we can simply take the data we’ve got and stack it up against a different fragmentation threshold value, let’s say, 1,900 ft/s. If we do that:

M16A2 (V0 3,150 ft/s): 391 m

M4 (V0 2,920 ft/s): 326 m

M16A2 (V0 2,950 ft/s): 335 m

M4 (V0 2,720 ft/s): 268 m

Immediately, we see that by changing our assumption for fragmentation threshold velocity, the entire ball game changes. Without questioning these thresholds for the time being, we see that a velocity-compromised M4 loaded with M855A1 has a 78 meter (41%) fragmentation range advantage over a brand new M16A2 loaded with hot M855 and fired under ideal conditions. The ideal M16A2 loaded with M855A1 enjoys a full 201 meter (106%) advantage over its counterpart loaded with M855.

So, in the end, what do we make of this? Well, it’s tough, and the only basic conclusions we can come to are “it’s complicated”, and “the M16 generally enjoys an advantage over the M4”. However, the most obvious conclusion is that if a longer fragmentation range is desired, the right answer is not a longer barrel, but a bullet that fragments at a lower velocity. Something, it seems, the Army has already realized.



Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


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  • iksnilol

    MY BOY NATHANIEL BRINGING THE FIRE, SUCK IT, VALORIOUS!

    ๐Ÿ˜€

    Also, I owe you a beer or drink of your choice, Nathaniel.

    • Be nice!

      But sure, I’ll take you up on that… Whenever I get out that way.

      • iksnilol

        ๐Ÿ˜› sorry, had to get it out of the system somehow xD

        I will be honest, hoped some more about bullet drop but that can readily be found online so no worries ๐Ÿ˜€

        But yeah, I do make a mean Pina Colada and I will probably go to the US in a couple of years when I am of legal drinking age there. So I’ll probably try to hit up your AO when I do that.

        • lol

          You’re just a baby, explains a lot.

          • iksnilol

            And you’re a diaper-head. Which also explains a lot.

  • zmd

    I never really understood why the U.S. moved away from the better-fragmenting 55gr FMJ round. Was piercing Soviet helmets at 600m really as important as getting reliable fragmentation effects from body shots at 200-300m?

    • Due to its inferior ballistic coefficient, M193 doesn’t really have a better frag range than M855.

      I’ve certainly heard lots of claims that it fragments more consistently, but never seen any actual evidence of that, either.

      • Porty1119

        I’ve heard that M193 out of a proper 1-in-12-twist barrel produced horrifying/excellent fragmentation, well in excess of any M855.

        • Both rounds can fragment to approximately the same degree (they have very similar jacket and core masses, and the same jacket thickness). It is a matter of how consistently they do so, and at what velocities. I’ve seen no real evidence of a difference between them in either respect.

      • CommonSense23

        Do a live tissue lab. M855 starts showing its problems in live tissue.

        • Mmm, that’s the trick, isn’t it? Where do I get all the goats?

          But yeah, live tissue testing is extremely valuable and accordingly extremely difficult. Shame there’s not more that’s public.

          • CommonSense23

            Every live tissue lab I have done
            Every PJ, 18D, surgeon who has dealt with who had real world experience with dealing with 855 says the same thing. M855 is just really unreliable.
            Normally I’m not one to lend credence to battlefield hearsay. But when you start hearing issues about a round for over two decades from multiple sources from grunts to SOF, to medics to surgeons. It starts painting a picture.
            I really have started believing ballistic gel starts having issues with high speed low weight projectiles. There are so many small variables at play with external to terminal ballistics. The only way to truly confirm a bullets ability is live tissue.

          • It’s not a secret that M855 is unreliable. What I don’t know is whether M193 is better in this regard.

          • CommonSense23

            I got limited real world experience with M193. But seen it in live tissue and it seemed to be consistent with its effects. Which granted were always from less than 10 ft from a 16inch.

          • That does not shock me, given the velocity. I suspect by 25-50 meters, you’re looking at consistency much more similar to M855.

          • CommonSense23

            The issue is at the same distances M855 was producing thru and thru.
            But honestly it’s all moot now with the widespread adoption of 855A1 and 318 for non SOF, and 262, 255, and 70gr for widespread use in SOF.

          • Joseph Goins

            “It’s not a secret that M855 is unreliable. What I don’t know is whether M193 is better in this regard.”

            You might want to forgo the M193 test. Recreational use that simply because it’s old and cheap. Soldiers are now using M262 in their shorter-barreled Mk18s. From what I saw in Iraq, that setup worked better within 200m than any combination of M193, M855, and barrel length.

          • I wasn’t really planning on doing an M193 test, honestly.

          • Joseph Goins

            “Where do I get all the goats?”

            I heard ISIS was having a fire sale.

          • ostiariusalpha

            Only slightly used.

          • Tassiebush

            And the sheep are slightly ewezed

          • Tassiebush

            I heard that one of them starred in an ISIS adult film called “Deep Goat”

          • Uniform223

            ISIS has other things in mind for using goats…

          • Joseph Goins

            Don’t worry. They are only selling the damaged ones.

      • n0truscotsman

        Like the M16, the M193 is also subject to a myriad of different myths as well. Interestingly enough, these myths seem to be overexaggerating its terminal effects.

        I just like it because its cheap range stuff.

    • Evan

      M855 performed better out of the M249 SAW was the main reason. XM193 just didn’t have enough mass to really push through “concealment”, meaning vegetation, etc. The Belgians specifically addressed the problem by adding a steel penetrator to the bullet 55 grain bullet, essentially creating M855.

      It’s a lot of conjecture, as even in the original documents from the 1970s, the lack of fragmentation compared to XM193 is considered “more humane”, whereas with our understanding of ballistics today, I doubt few knowledgeable people would argue that.
      Keep in mind, M855 supposedly performs better than 7.62×39 M43, but both M193 and and M43 have killed thousands.

      As Nathaniel points out, a lot of that old conjecture has turned out to be old “Fudd-lore” and hokum. Just as gun manufacturing techniques have improved since the 1960s, so has our understanding and abilities with making better ammo, hence Mk262, Mk318, and M855A1.

      On the flipside, there is no magic caliber that can do everything well at every range. We’re just trying to make a round that does a little bit of everything well, for ease of logistics and cost concerns.
      This is the whole reason for the advent of the “Assault Rifle” and the abandonment of combined arms squads of late WWII and shortly after with a mish-mash of rifles, carbines, SMGs, etc. and other special purpose weapons of various calibers; which again, was the whole genesis for NATO-standardization.

      • At the time, some of the marketing got pretty schizophrenic about SS109, at once claiming that the new round was “more lethal” but also “more humane”. I think what they had to say mostly depended on who they were talking to.

        I really wouldn’t expect to see much of a difference between M193 and M855 in a controlled experiment with a large sample size, with regards to fragmentation range and consistency. Which makes M855 essentially the better round, since its long-range performance is probably better.

        This is all pretty much academic, though, since M855A1 exists.

  • MrBrassporkchop

    I was shot once but it’s okay because it was from a 14.5″ barrel.

    • Patrick Selfridge

      Buuwuuaahhaha!

  • Black Dots

    Don’t use M855 for self-defense. Got it.

    • Henry Reed

      The 77 TMK apparently has some devastating terminal ballistics, with all the long range benefits of the Mk 262

      • happycamper

        It most certainly does, even out of short barrels or at extreme range.

        Ive hit groundhogs and coyotes out past 400 yds with the 77gr SMK from a 18″ Douglas SPR barrel just like in the Mk12 and the effect is pretty dramatic, If you are interested in keeping a pelt I suggest you avoid the 75 gr Hornady and 77gr SMK.

        One thing that I have noticed with the SMK’s is they MUST have the cannelure and it needs to be deep. Without a deep cannelure Sierra is right the Match King line is not good for hunting, it does not fragment.

        Ive tested the 68 and 69gr SMK’s too any they need the deep cannelure to perform well.

        All of the Heavy Match Hornady ammo performs well in tissue.

        • Henry Reed

          I was talking about the Tipped MatchKing, which adds a ballistic tip to the SMK and not only increases the BC but creates more violent and reliable fragmentation.

    • Joseph Goins

      When I was getting out of the Army in 2006, we started putting MK262 in our Mk18s. The effect on flesh (0m-200m) was a lot better than M193 and M855 regardless of barrel length. I still buy it and use it in my SBR.

  • User

    Good article! With M855A1 barrel lenght is far less a “problem”. The thing is rifle powder is so insanly degressive burning that the most acceleration is near the throat anyways.

  • AC97

    20 inch barrels are still: smoother shooting, cause less of a muzzle blast, and are still, with an A2 stock, about as long as an 18.5 barreled shotgun.

    Bore pressure as bullet uncorks:

    20″ – 5,717 psi
    14.5″ – 8,150 psi
    10.5″ – 11,500 psi
    7″ – 17,040 psi

    Source: http : / / www (DOT) sadefensejournal (DOT) com /wp/?p=1093

    (Remove spaces and replace “DOT” with a “.”

    • I linked that article in the text.

      And for the record, I am not (and have never been) “anti”-20-in barrels. I actually am quite fond of the idea of an “ass-end” carbine with a 20″barrel, like the C7.

      However, I don’t think short barrels are going away, either.

      • It would be cute if some manufacturer introduced a C7A2-like gun and called it the “Ascend Carbine”. I would giggle.

        • 40mmCattleDog

          I have the VLTOR A5 buffer and tube ony my 20″ and i gotta say the collapsable stock works magic for making a 20″ much more handy.

      • AC97

        I personally think 20 inch barreled ARs are cooler (hence the reason I got one).

        I agree, short barrels are not going away, period. That being said, they’re somewhat overrated for civilians, who probably aren’t going to be clearing rooms (but would blow out their eardrums with a 10.5 inch or 11 inch barrel without earpro).

        • iksnilol

          That’s a bit too short tho ๐Ÿ˜› I think he meant more in regards to the classical 14.5 M4 barrel (which is pretty nice for us civvies if you pin a flash hider with QD suppressor mounting).

        • Joseph Goins

          Hey hey! Nutnfancy said your gun needs to weigh -4.7lbs. You can’t afford those extra 9″.

      • roguetechie

        LOL,

        The 20 inch bbl with 6 position stock setup is how I prefer to configure my AR’s!

        Put another way, your charts definitely indicate that ammunition choice and budget needs to be a factor when you’re deciding how to configure a gun!

        I try to explain this to people.

        Yes, AR’s are like big boy Barbie’s, but physics doesn’t care how badass your gun looks.

    • iksnilol

      I dunno, I think the 14.5 would be softer shooting with a suppressor, and shorter as well ๐Ÿ˜‰

      (that is if you use a suppressor that extends 10 cm and not 20 like most American suppressors do. I am not entirely unconvinced that modern American cans are repurposed car mufflers).

      • Paul White

        I’ve gote one with a 20″ barrel and one with a 14 with the stupid birdcage; the 20″ is *much* more pleasant to shoot.

        • iksnilol

          I was thinking 14.5 + suppressor/silencer/moderator is more pleasant than 20″ with only flash hider.

          • jono102

            A good quality 14.5-16″ rifle with a good quality suppressor will IMO always shoot nicer than a 20″ without. With suppressors being unrestricted here for civilians, you’ll most likely encounter 14.5-16″ AR’s vs 20″ apart from Long range shooters and Service Rifle competitors.
            From a military perspective where suppressors are becoming a lot more common, a 20″ rifle with a suppressor is getting pretty long.

          • iksnilol

            Something like that.

            Here in Norway I find longer barrels to be directly uncommon, even if it is cartridges that “need” longer barrels such as 30-06 or 6.5×55. But the kind of suppressor is also important, I mean, in Europe most suppressors add about 10 cm in length (4 inches) + equally much behind the muzzle. For cans not going behind the muzzle the longest that I’d find acceptable is about 15 cm (5.5 inches or so).

        • Uniform223

          Get a mid length gas or one that has one. Too bad the Army passed up on the M4A1+. If I remember correctly they were going to put a mid length tube on the barrel.

          • Kivaari

            A 16″ mid-length carbine seems to be about ideal from my perspective. I have one with a BCM lightweight upper as well as a Colt M4 11.5″ variant. The 16″ carbine IS a better choice. Both are equipped the same way as far as optics and BUIS and I can’t tell the difference in how they handle. Since they handle the same, the longer barrel just makes sense and doesn’t need ATF paperwork.
            I should have saved my money.

      • Kivaari

        About 1/3 of the OAL of QD cans do nothing but provide for the attaching mechanism. A very effective direct thread can is easily a third shorter than the QD cans, and works just fine all at less weight and bulk.

        • iksnilol

          I’ll admit, I’ve not much experience with QD cans. Only one I know of that’s somewhat available here in Norway is the Ase Utra designs. And the QD version of that was about 2 cm longer but also added 2 cm less to length. Though you do need to use their BoreLock flash hider with it.

    • George

      As noted on earlier posts, expansion ratio, expansion ratio, expansion ratio. .300 BLK has about 2x the barrel cross section area as 5.56, so a 10″ .300 has the same barrel volume as 20″ 5.56 does.

      With sound everywhere and particularly enclosed spaces an issue, something to think about. Not just suppressor users.

      (That said, five quatloos to someone who invents a .308ish alternate cartridge that packs in AR mags ok and won’t let a 5.56 gun chamber it…)

      • ostiariusalpha

        I sort of layed out a preliminary concept to Nathaniel in a previous article comment: using a 5.45x39mm as the parent case, necked up to a .308 bullet, would do the job admirably. There is 5.45 Soviet brass out there to experiment with, and I’m considering a custom die to test the possibilities myself for fun. Ideally, the cartridge would have a rebated rim so that it could use the standard .223/5.56 bolt, giving it all the same advantages as the .300 BLK, but without the danger of chambering in the wrong gun.

        • ARCNA442

          Maybe I’m missing something, but wouldn’t necking up 5.45×39 to .308 basically just be reinventing 7.62×39?

          • 5.45×39 and 7.62×39 have different bases.

          • ostiariusalpha

            No, they are only the same case length; their case heads and rims are different diameters. 7.62x39mm has a .447″ rim, and 5.45x39mm has a .394″ rim, which puts it much closer to the .378″ rim of a 5.56x45mm cartridge.

        • George

          Nice. Missed that earlier.

          Can test with 5.45 bolts. A modified rebated case for 5.56 bolts is easy to run on a lathe but hard in quantities 100s for testing.

          Cartridge case tolerances should zero chances of chambering the Wrong One. This .308 Abominator won’t fit in a loose very worn 5.56 deep enough for the bolt to rotate and close. 5.56 in its chamber is just too long to the shoulder. Nice.

    • n0truscotsman

      Addressed by:

      1.) A mid-length gas system
      2.) Suppressor

      As much as I like the nostalgia that a 20″ barrel brings…

  • Wolfgar

    It would be interesting to see how the MK262 and other rounds performed and if there would be any significant difference than the M855. The 5,45X39 would also be a good comparison.

    • Military 5.45×39 doesn’t fragment, so it wouldn’t register here.

      • Wolfgar

        True but it does tumble very quickly which I guess would be a terminal comparison at range. My mistake.

        • M855 also tumbles, and we don’t see “tumbling thresholds” like we do with fragmentation. Rather, bullets tumble less and less readily as velocity decreases, but they still always tumble.

          • Wolfgar

            We do see certain bullets up set quicker compared to others resulting in a more effective wound. Comparing these rounds at distance would clarify a lot of controversy if any.

          • Sure, but that’s not something I can do using the data I have. One would need to conduct extensive empirical tests.

          • Wolfgar

            I thought you could do anything ๐Ÿ™‚
            Thank’s for your time Nathaniel and have a happy Thanksgiving!

          • Well, for the record I am moving in a more experiment-driven direction these days, but it won’t happen overnight. Playing with spreadsheets is fun and all, but I need to put out some data of my own.

            And happy Thanksgiving to you, too!

          • noob

            yay gel videos!

          • DanGoodShot

            I wounder if there would be any way of getting cadavers?? Hey, it is for science! Lol

          • If anyone wants to send me some gel, sure. Although articles are more my speed, not videos.

          • Matt

            Nathaniel, if you want to come up to Iowa, a buddy and I are working on setting up gel test videos. We are also 30 minutes drive from Gelita, so real gel is available.

          • I appreciate the offer. Right now I am juggling a few things, so I can’t promise anything but I’ll certainly consider it.

            Also, have you seen Joerg Sprave’s video on Gelita?

      • roguetechie

        Nathaniel,

        Some people swear that polygonal rifling like HK uses gives a minor to very significant MV bump in a given barrel length.

        Is this true at all?

        Or is it just marketing fluff?

        I’ve always kinda wondered.

        • It would be very hard to prove, as the reason it would give a MV buff is better sealing around the bore. So one could just as easily create a traditional rifling configuration that would probably do just as well.

          However, I have seen some evidence that Glocks generally produce higher MVs than other guns with similar barrel lengths, so there might be something to it.

          • roguetechie

            I remember when 416 mania was first hitting that there were people out there claiming up to +300 fps from a 16 inch barrel.

            I’ve also seen evidence various places for the glock velocity increase.

          • A 300 fps increase is absurd. That’s a ~20% increase in efficiency just from the rifling configuration!

            If we’re talking a real increase in velocity, it’s more on the 20-50 ft/s range, tops.

          • roguetechie

            Yes it is absurd, but if you go back and read arfcom at the peak of the 416 fever people actually claimed that.

  • Ed

    Overall depends on the type of combat you’ll be in. In open forest of Europe or Afghanistan a M-16A2/4. If you fight in Baghdad or Benghazi a M-4 gets the nod. In the end be better for the soldier to chose which 5.56mm weapon he wants….. Also M-193 ball was always better to tumble and fragment than 855. Overall it was false fears of Soviet body armor which created M-855 in the late 70s.

    • The_Champ

      Yeah I wouldn’t mind seeing Nathaniel work up some numbers for M193 as well just for fun.

      • Alright, here’s something to chew on, M855 vs. M193 from 20″ barrels. This is assuming the same “new” condition gun with hot ammo (keep in mind that M193 is all over the place for velocity, you can get anywhere from 3,150-3,400 ft/s from a 20″ barrel, depending on the mfgr):

        http://i.imgur.com/BJkkJTZ.png

        M193 is on there twice, because there are several different variants of 55gr FMJs out there, as well. A lot of M193 will have the dumpy “potato” bullet typical of, say, Federal, but something like PMC will have a much sharper contour and correspondingly higher BC. Worse, in some cases you’ll see bullets across this spectrum loaded into ammunition from the same lot! There is a ton of inconsistency here, so I just covered the two extreme ends of the spectrum with one using 0.120 G7 BC and the other using 0.145 G7 BC. IMO, if you want the best M193, IMO, PMC XP193 is the best stuff from a ballistic perspective. They use a very consistently shaped bullet (which I suspect is their own home-grown derivative of the Sierra 55gr) that will be on the higher end of the BC spectrum, and they produce very high velocities from 20″ barrels.

        So, then, we see that versus the 190m figure for M855, low end M193 is getting only 179m of frag distance, while the finer-shaped stuff can get as high as 216m. Pretty much the same, in other words.

        • The_Champ

          Very interesting. Thanks for the quick response.

          I think this came up in another conversation we had, and if I’m remembering correctly, you’d didn’t give much credence to the idea that the twist rate in the barrel effected bullet tumbling. Is this correct?

          • It definitely does affect it, and it probably doesn’t affect it. I’ll explain: In my article “The New Caliber Mafia”, I wrote this

            “The twist rate of the barrel helps determines the stability of the projectile through media, in this case air. A tighter twist rate will better stabilize the projectile, reducing the precession of the bullet (the degree to which it deviates axially from the flight path). It is this reduced angular deviation that can cause through-and-through wounds, not the bullet being stable through flesh. In other words, a bullet stabilized by a 1-in-7 twist rate barrel may hit the target at a shallower angle and thus yaw later than one stabilized by a 1-in-9 twist rate barrel. I highly suspect this is why you will be hard pressed to find a gel test video online of M855 being fired from a 1-in-9 twist rate barrel and failing to upset within about the first 5”.

            “Somewhat paradoxically, this tight twist rate should give M855 exceptionally consistent long-range terminal effectiveness. The same excellent stabilization that minimized precession also ties the bullet more closely to its original orientation through its flight. That means that at long range the bullet is flying through the air at an upward angle relative to the arc of its flight. If it hits a target at this angle, it should upset readily and tumble within the first few inches of tissue.”

            This is true because it describes the physics of projectile stabilization, but it’s not really accurate because it doesn’t account for fleet yaw. In other words, fleet yaw should mask the difference in stabilization between different twist rates. However, fleet yaw eventually dampens out, so then the question becomes: Is there a distance range where a bullet has overcome its fleet yaw, but is still only marginally stable through one twist, while fully stable through another?

            It would be very difficult to figure this out, and I suspect that it doesn’t matter. Regardless:

            – The idea that twist rate does not affect yaw because no barrel could possibly stabilize a bullet through tissue is WRONG because the neck length of yaw in tissue is affected by the angle of attack of a bullet, which may vary with different twist rates and spins.

            – Twist rate probably doesn’t affect yaw in any practical sense, because it will be masked by the turbulence produced at the muzzle which creates fleet yaw effects.

            I also want to note that this is just my read of the information I know. I might be wrong.

          • The_Champ

            Good food for thought, thanks!

    • Ed

      Yeah get a M-16A2 and a M-4 with 1 in 7 twist barrels and experiment with XM-855 and US XM-193 by Federal some IMI 193 and some PMC South Korean 193 make some video about the results. Be a good 2-3 days of TFBTV.

  • lowell houser

    I’m one of those people that don’t get the need to turn a 5.56 into a pistol with a shoulder stock. At all. The cartridge is already heavily bullet dependent, why you would want to slow it down that much on top of that I have no idea. Not volunteering to be shot with one, but there it is.

    From what I can tell, 7N6 had M855 beat in every way before M855 was even conceived with it’s “punch through sheet steel at 600 meters requirement.” M855A1 just leap-frogged over it. I bring it up because the Russians seemed to think that a 16in barrel sending a 7N6 bullet at a measly 2900ish was just fine. We keep arguing over which is better when maybe we should remember some words of wisdom from the racing world – it’s not how much horsepower you have, it’s what you can put on the pavement. That 14.5 in barrel is as low as you probably want to take it, but load some 75gr hollowpoints with a 1/7 twist and you’ll probably be okay.

    For those that just absolutely have to have a super short SBR, I recommend InrangeTV’s latest vids from the Red Oktober match – Ian’s pseudo-krink was a liability. He gave up velocity and all he got in return was extra concussion and a ridiculous fireball out the muzzle. Fun gun apparently, but that’s all it is.

    A moral to all my ramblings? Carbines are the sweet spot. Yeah. we’ll go with that.

    • Dracon1201

      Ian was using 7.62×39 as well. That makes a big difference when watching both runs, Karl is also a very good competitor by comparison… Just saying…

      • tts

        Even Karl seemed to think Ian’s gun wasn’t very competitive for the match though. Its very clearly a gun meant more to have fun with than be practical.

        The sight radius difference alone seemed to be a big hinderence. Especially at night.

        • Dracon1201

          He also didn’t use any modern optics, which render length as a moot point. Backup sights aren’t the best. I’d put up a modern SBR AR against a 16″ AR and bet the times would be very similar to each other with optics.

          • tts

            Sure and if he’d had a suppressor too the muzzle blast wouldn’t be a issue either.

            But he didn’t have optics or a suppressor so the ‘woulda coulda shoulda’ stuff doesn’t matter here.

          • Dracon1201

            Well, yes, but that’s exactly the same apples to oranges comparison between mismatched calibers and weapons systems that you were trying to describe. In short; it’s much different.

          • tts

            Its a match where you bring anything combloc/AK to compete and not meant to be a apples to oranges comparison.

          • Dracon1201

            Looking at two entirely different calibers pitted against each other creates that. Especially when you are trying to make a comparison to 2 5.56 systems…

          • tts

            Yes I know what a apples vs oranges comparison is but that wasn’t what was going on at all. You’re missing the point or reading into things too much.

    • Tassiebush

      I just cracked up over Ian’s costume!

    • happycamper

      The Soviet 5.45×39 is a better designed projectile for a small caliber military round for short barreled weapons because it uses a long for caliber ass heavy bullet that will tumble as soon as it hits anything.

      The 55gr 5.56 XM193 is still a great round, but it needs to be out of a 16+ inch barrel to get that devastating terminal effect at range, when all the M16’s were 20″ guns who cared, it was a 500yd capable weapon.

      But cut half the barrel off double the rate of twist and you have a gun that isnt designed for your bullet. Yeah M4 sweet!

      The M855 is crap, they took a bullet thats primary wound mechanism is fragmentation and gave it a thick jacket and only cared about punching through a soviet helmet at 600m. Yeah because that is where most combat happens at extreme range with headshots.

      There is nothing wrong with the 5.56 the 77gr SMK is one hell of an effective round (in military form with deep crimped cannelure) and instead of blowing billions on the BS M855A1 we should have made a low cost analogue for the 77gr SMK, Hell we could have just bought a few billion full match grade loads from Sierra and come out ahead.

  • Bullphrog855

    The question of the hour is… why not both? The Hi-lo philosophy is a proven one, I don’t see why we should pretend the hi part of the equation doesn’t mater.

    • Uniform223

      The “hi” is covered by DMRs.

      • Bullphrog855

        It would be if they issued them to every other soldier in the platoon. No, it’s just a niche.

  • A Fascist Corgi

    You continue to advance the narrative that the standard infantryman shouldn’t have to worry about engaging in mid- to long-range combat with his assault rifle. Barrel length and velocity doesn’t just affect fragmentation thresholds, but it also affects bullet drop and wind drift.

    If NATO adopted a new round that was even more sensitive to barrel length, then the re-adoption of the 20-inch barrel would be a no-brainer. The more I think about this problem, the more I side with long-range performance over recoil and weight. Combination muzzle devices that both reduce muzzle rise/recoil and muzzle flash would offset the additional muzzle rise/recoil created by the adoption of a more powerful round – which would still allow for controllable full-auto fire.

    NATO should adopt a new assault rifle that’s centered more around long-range performance. That doesn’t mean that NATO should go crazy and adopt .308, but something in-between 5.56 and .308 should become the new standard issue NATO round.

    • This post only looks at fragmentation threshold and terminal effectiveness. I never said velocity didn’t affect bullet drop, and in fact covered that aspect very thoroughly in the 20-something posts I did as part of the Modern Intermediate Calibers series.

      “Do you really need full-auto fire for close-range combat? Does anyone feel underarmed with the semi-automatic AR-15 that they’re using for home defense.”

      I would absolutely feel under-armed with a semiautomatic AR-15 if I was defending my home against ISIS.

      “NATO should adopt a new assault rifle that’s centered more around long-range performance. That doesn’t mean that NATO should go crazy and adopt .308, but something in-between 5.56 and .308 should become the new standard issue NATO round.”

      That’s one perspective, but if you’re going to go down that route, you’ve got to commit with training and optics and reorganization. Which means $$$, and most NATO countries do not have a lot of that.

      • CommonSense23

        Saying you would feel under armed without a select fire weapon against Isis or the Taliban is kinda pushing it. Probably the most common mod I have seen done to MK18s after grip and stock is swapping out the trigger for a better one. Which leaves its semi only.

        • It’s all about feelings. ๐Ÿ™‚

          My point was that what is appropriate for home defense is not applicable to what is appropriate for a service weapon.

          • CommonSense23

            I kinda feel that at this point. We have hit the point where one rifle can be the best for both groups. A 16inch 5.56 semi auto rifle is pretty much the sweet spot. It gives you a weapon that needs minimum training to be proficient compared to others. Fires effective rounds, lots of capacity, low recoil. The needs of a service rifle will undoublty change in the future. But I feel right now with the weapons on hand the service rifle and H.D. factor go hand in hand.

    • Uniform223

      And all this just oozes with your experience watching videos of TICs?

    • Joseph Goins

      Why? Most engagements are within 200 meters. I think you might need to rethink that idea.

    • n0truscotsman

      You *do* realize Nathaniel has already addressed the ‘intermediate/general purpose cartridge’ argument, right?

      If you were going to hypothetically do this, you need to figure out what your requirements are, how your infantry are going to apply them in differing combat environments, and what equipment/assets they will have available for support. And probably two dozen other questions I haven’t mentioned so far.

      Because unless you start literally from the *ground up*, you are merely chasing Increasingly expensive diminishing returns.

      “Barrel length and velocity doesn’t just affect fragmentation thresholds, but it also affects bullet drop and wind drift.”

      Is the M4 measurably *worse*? As in *worse* enough to profoundly affect the combat performance of infantry platoons? If you are asserting this, what data do you have to back those conclusions?

    • CommonSense23

      You realize that mortars, rockets, belt feds, guided munitions, arty, and ca s exist for a reason. It doesn’t matter what weapon you give the infantry. The infantry is never going to be able to reliably engage past 200 yards with a rifle when trading effective fire.

  • Gary Kirk

    Have built, carried, and used both. Personally prefer the M-16A4, but that’s just a personal preference, doubt any real world “operator” (hate that word) could really tell the actual difference.. Both have advantages and disadvantages, I say let the end user decide what they like, and what works for them.

    • Gary Kirk

      Also, just for the record, I hate collapsing stocks, understand the want/need for most people. They just don’t work for me..

    • Joseph Goins

      I couldn’t. After 200 meters, it makes a sterile hole. I was happy with the M4 when we had to do room clearing and convoys. I would prefer the M16A2 if I were fighting in an open field against Russia.

  • ๐Ÿ’๐Ÿ‘Š

    We need a chart, with observed or known fragmentation thresholds of various projectiles in various rifle calibers. FOR SCIENCE.

  • John

    Use 16″ barrels for a happy medium. Got it.

    • tts

      18″ barrel Dissipators would be a happy medium IMO.

      16″ barrel carbines are a still reasonably performing compromise biased towards a shorter barrel length.

      14″ and less probably aren’t worth it without special ammo tuned for that short of a barrel chambered in 5.56.

  • JSmath

    The actual tendency to fragment, in my opinion, would be dependent on metrics far more specific than just general velocity. The 2,500fps mark might be spot on for M855, but off for M855A1, but there isn’t any reason to assume it’d be nearly identical to M80A1. Different dimensions, twist rates, section densities.

  • Ron

    Fragmentation is a bonus when you get it, but all and all is not required for lethality. The number one requirement is hitting the target and hits to the high chest area tend to be debilitating if not fatal even if they only produce “ice pick” like wounds.

    Marines and SOF have found that they have been able to kill with 5.56mm from SAMs and Mk12s at the 6-700 meter range, well beyond the point the round is fragment

    • jono102

      Exactly what I was thinking, putting the round (shot placement) where it will be most effective i.e. kill areas etc is the first and foremost a consideration in a military context.
      Where, when or why fragmentation may occur has such a massive variation to it, that is something that doesn’t even pop up as a consideration to a soldier.

  • Vitor Roma

    The Nosler RDF seems it could turn the M16 in a solid 800 yards gun. Higher BC than the mk262 while being faster.

    • CommonSense23

      The M4 can easily reach 800 yards with 262.

      • Joseph Goins

        I shot at someone in Mosul from a Mk18 with M262 at a distance of ~600 yards. When we conducted the battle damage assessment, we found him dead with one round in the chest cavity. The guns are capable of more than we give them credit for.

        • iksnilol

          But, but…. muh M16 superior range?

          Couldn’t you have missed on purpose or somehing to maintain that narrative?

    • I don’t for a second trust Nosler’s BC figures, especially not when they’re giving me an 0.945 i7 FF for a bullet with an <0.50" ogive length.

  • Joseph Goins

    From my personal experience overseas, there is no significant difference that should cause anyone to question a 14.5″ barrel. I wouldn’t want to be shot by either.

  • Tierlieb

    Thanks for looking at it only for 14.5″ and 20″, making this a military comparison. It seems several commenters missed that: It is the equivalent of debating what FMJ round is the best for a pistol.

    It does not concern civilians, who can simply use a Federal TBBC, Winchester Bonded JSP, Barnes TSX or any other round that does not rely on fragmentation.

  • Tassiebush

    I just find it fascinating that even with such a short barrel a round that relies on good velocity to kill can still perform.

  • LOgistical

    Have wondered if, or if not – why not, barrel length is chosen on a real scientific basis.
    Why seemingly always full or .5 inch increments? Or 0 or 5 ending if designed in metric. What if 17.38″ or 137mm(pistol) is the “best” compromise? “Full” measurements mean nothing for QC, it either fits the gage, or no.
    A secondary issue here I suspect is cost. Assuming 100,000 units, the difference between M4s and M16s is over 7 miles of machined steel, chrome plating and handguards.
    And the subsequent fuel costs of delivery & alllllllll the “potential” subsequent transportation hither and thither of that weight difference.

    • iksnilol

      Well, do you want to test a barrel and cut it in intervals of 0.01 inches? Heck, you wouldn’t need to cut, you could just grind it shorter. It’s only 2.5 mm.

    • Kivaari

      Input from the soldiers. They are questioned about what they want in a rifle. The M4 exists because many desired a shorter rifle. The carbine length system existed since the 60s., adding a 14.5 inch barrel to the carbine system meant a bayonet fit. If they chose a 16 inch barrel they would have needed to re-invent the thing and develop the now popular mid-length system. Just look at history and see how infantry rifles changed from 1890 to 1920. They got shorter because the troops asked for a shorter rifle. The Soviets kept long barrels “longer” than everyone else of significance, but by 43 the M44 was coming on strong. Troop feed back, even in the Soviet Army had an impact. Troops want a handy rifle. If I were in charge, knowing what I know now they would be issuing 16″ mid-length carbines, but that would cost a bunch to re-arm everyone with such a gun. But, it sure could be phased in by just buying new uppers. And the bayonet would still fit.

    • Barrel length is often determined by other factors, and the M4 is a good example of this: It has a 14.5″ barrel because that is the length you get with a carbine length gas system and enough barrel sticking out the end to fit M16 bayonets and other accessories.

  • Klaus Von Schmitto

    The data seems to make me look sort of smart with my decision to go with 18″ 1/8 twist barrels on all my AR’s except my shorty. And they make moot the 5.56 vs .223 debate with the Wylde chamber. I think.

  • Excellent post Nathaniel. Your scientific approach is spot on. I really believe you nailed the conclusion when pointing out the best determination of bullet efficiency is the cartridge itself and not necessarily the length of the barrel.
    Obviously a longer barrel does give greater velocity at greater ranges, but proper ammunition selection for a shorter barrel can be highly effective as well.
    Like almost everything firearms – the decision comes down to the end user and their particular needs, and limitations.

    Great post!

  • claymore

    Very nice work and presentation. Great to see you using actual FACTS to make your case unlike the usual anecdotal blah blah “It’s better because joe billy bob told me…..”

  • Bradley

    I wish they would make 223/556 loads that used an extremely heavy, pistol shaped hollow point for close quarters use through short barrels. Yes you could just use a more suitable cartridge, but this way you could still use normal 556 if you needed to.

    • CommonSense23

      Fun fact they do.

      • Bradley

        Example?

        • CommonSense23

          Blackhills 70gr.

          • Bradley

            I’m thinking 100+ gr flat/round nose hollow point. A Spitzer bullet that heavy, even if it fit in a magazine, would be so long it would need an extreme twist rate.

          • CommonSense23

            But why. 70gr Black hills is already designed for 10inch barrels, expands past 200 yards from a 10inch. It was specifically designed as a expanding 5.56 round for 10inch barrels.

          • Kivaari

            I’d have to support CommonSense23 on this issue, WHY? If you can’t get good results from a 70 gr. quality expanding bullet at 25 feet range just what would a 100 grain bullet do better. You may as well shoot a 110 grain 300 BLK at supersonic velocities if you are worried about close range work. But personally I see no need for ME to have a .300, since a 55 JHP will rip anything to shreds.

          • Bradley

            Well I can’t really say since I’m not an expert like you. I don’t understand scientific concepts such as “rip anything to shreds.”

  • That’s not exactly an independent test, that’s a guy who’s estimating BC via bullet drop.

    When I see doppler results, then maybe I’ll believe it. That’s an extremely high BC for such a small, modestly shaped bullet.

    • ostiariusalpha

      I’d say it’s independent, just not rigorous enough to be particularly useful. Mr. Bentham mentioned nothing about velocity measurements or atmospheric data, meaning he probably didn’t take any. He seems to have just noticed that he and his companions were shooting a little high and assumed it was entirely due to the bullet’s ballistic coefficient being higher than advertised.

  • LazyReader

    A heavy bullet in a short barrel is good, that’s why .300 BLK is becoming popular.
    but if your still using 5.56 were lucky there are 16-18 inch barrels for those that don’t want that 14.5 inch impediment

    • Kivaari

      Except all these people want is a 500m rifle and it wont be had in a 300 BLK. The solution for any “shortcomings” of the 5.56mm in the civilian world is to shoot ammo like TAP. How many of we civilians are ever going to get in a shootout that involve ranges much over 25 feet?

  • NewMan

    You should also talk about all the compromises/sacrifices made when they shorten the M16 into a carbine, with regard to reliability and durability.

    • These days there really aren’t any. It’s well known how to make a 14.5″ gun run.

      • NewMan

        The M4 has significantly shorter parts lifespan than the M16. It also requires heavy buffer and Oring for proper function. But even then those are bandage fixes, not solution.

        The carbine gas system is meant for 11 inch barrel, not 14 in. It was a major compromise to make it works with 14 in.

        The M4 is very over gassed, high pressure and thus put a lot of stress on critical components

        • How are those not real solutions? Oh, right, knowing you it’ll be because they didn’t stuff an oprod in there.

          Improving the MPG mass, extractor tension, feed angle, and tweaking the gas port don’t sound like “band aid fixes” to me, they sound like “how you get a gun to work”.

          I mean, you like the HK416, right? How did they get that gun to work? Well, they overgassed it and strengthened all the springs, and lo and behold it has a higher parts breakage rate than the M4. Does that make the 416 just a bunch of “band-aid” fixes? Does that make it a bad gun? No, absolutely not. It just makes different trade offs to get a slightly different result.

          And anyway, saying that the carbine length gas system was “meant” to be used with an 11″ barrel is about as meaningful as saying that Sally and Pete in 5th grade are “meant” to be together. Which is, basically nothing. Sure, when you’re tapping gas that close to the muzzle, you’re tapping at higher port pressure, and the system needs some adjustment to compensate. But it turns out that this isn’t that big of a deal, and that loads of people get even 16″ guns to work just great with carbine length gas systems.

          • NewMan

            The 416 is the solution to the M4’s many flaws. That is why the 416 has so much wide spread usage around the world.

            And that’s why midlength gas is so common now. The M4 is a flawed design. Midlength is so reduce pressure and smoother function. That’s why midlength guns don’t need heavy buffer and O ring. Strongly suggest u do an article on fatal flaws of carbine length gas system

            Whatever the case the 416 is the solution to all the painful problems that the M4 is having. You can’t make an DI AR to be as good and reliable as the 416

          • “The fatal flaws of the carbine length gas system”? Come on, you must realize this kind of hyperbole isn’t doing your argument any favors.

            How is the HK416 the “solution” to the M4’s flaws if in your opinion the M4’s flaws are lower parts durability and lower reliability than the M16, and the HK416 scores worse in the former respect, and hardly any better in the latter respect?

            The HK416 is selling well because it’s a good rifle marketed by a skilled firm that is willing to do what it takes to win contracts. Not because it’s the magical cure for every made-up myth about the M4.

          • NewMan

            LOL u show your anti-HK and anti-416 bias there bro. Look, the 416 is being used by countless of users out there and they have nothing but praise for the system. It is a quantum leap over the DI AR. The Marine are VERY happy with their M27 saying it is a huge improvement over their M16. U need to take a closer look at the 416A5 – the latest variant where HK even further enhanced the system.

            The 416 popularity is increasing everyday and is the standard by which all AR platform are measured by. And extremely high standard.

            DI is a dying breed.

            Look, u should do us a favor by writing a detailed article on the major short comings of the M4 system, the compromises that need to be made to shorten the M16 into a carbine, all the bandage fixes etc…

          • Alright, well I think everyone knows how you feel about the 416, so there’s not really a point to continuing this discussion.

          • n0truscotsman

            The 416 is used by many NATO member nations, as is the C8 and M4 (the later serving in more). So what? Many of those nations also use G36s, which are overrated.

            The 416 is essentially a M4 design with its own quirks and the G36 short-stroke gas piston system. Nothing that hasn’t been tried before. http://i295.photobucket.com/albums/mm155/markrsims/Rhino%20pictures/piston1xc0.jpg

            Certainly not a ‘quantum leap’, but rather, a modern reintroduction of a previously abandoned concept.

            the 416 is not ‘more reliable’ or ‘more durable’. The evidence that exists doesn’t demonstrate that.

          • CommonSense23

            So what are your actual firearms experience and training. And what articles have you wrote.

          • 22winmag

            Let’s call it what it really is. Internal piston… not DI.

            Open the hood on a Hakim if you want to see direct impingement.

    • jono102

      Like the 416

    • Uniform223

      You’re still here?

      • jono102

        AR’s were mentioned, It was only a matter of time before he arrived.

  • DW

    Long one. To compensate it being a .223. Not for ballistics though.
    ( อกยฐ อœส– อกยฐ )

  • V Mudde

    Basically, an M4 is like shooting an M16, but you have to stand a bit closer (~65 meters, in fact).

  • The survey of soldiers your referencing was conducted in part by Major Dave LaFontaine, whom I recently interviewed for another project. Yes, contrary to the narrative (which is driven by $$$), the M4 is very well liked by troops.

    • Kivaari

      Nathaniel, I hope you do an in-depth report on this report. The M4 is a fine weapon and it doesn’t deserve the bad mouthing it gets in the popular blog world. The HK people seem to be the worst offenders. It’s “German” seems to make some people think they are better. It’s German is BS to me.

  • BeenThereDoneThat

    You know, almost all of this is irrelevant to the average person/user. The professionals are, primarily restricted, right or wrong (assuming there is a right/wrong) to what they are issued/approved. The only ones arguing for/against are the armchair, mommies basement, game console bound and other assorted theoretical practitioners of the martial arts! And 90% of those that do take things serious can make do with whatever platform comes along! All of the “good” of one platform/feature over another is irrelevant since the dynamics of a REAL battlefield shorten the attention span of getting the job done!

    No plan survives first contact with the enemy!

  • uisconfruzed

    SBR AR–> 300BLK
    Out there AR–> 6.5 Grendel