Modern Intermediate Calibers: Trade-Offs – Bullet & Bore Diameter

Different bore diameters give rounds different properties. Despite the fact that all three rounds shown here - 7.62x39, 5.6x39 Russian, and 6.5x38 Grendel (two on the right) - all use the same case base and have virtually the same case capacity, they have very different ballistic properties due to their different bore and bullet diameter.

Different bore diameters give rounds different properties. Despite the fact that all three rounds shown here - 7.62x39, 5.6x39 Russian, and 6.5x38 Grendel (two on the right) - all use the same case base and have virtually the same case capacity, they have very different ballistic properties due to their different bore and bullet diameter.

Probably the most obvious element of ammunition design is the choice of caliber, or more specifically the choice of bore and bullet diameter. These two dimensions are of course closely linked in conventional ammunition systems (they can be decoupled with sabots, but those are outside the scope of this series), and together they relate to some of the most central trade-offs of any ammunition system design.

Simply, one may choose either a larger or a smaller bullet/bore diameter, with different consequences for either. With larger bore diameters:

  • Bullet mass, for a given scaled projectile design, will be higher.
  • Swept volume in the bore, for a given barrel length, will be greater.

Whereas, with smaller bore diameters:

  • Muzzle velocity, for a given scaled projectile design, will be higher.
  • Cartridge weight, for a given scaled projectile design, will be lower.

Therefore, smaller diameters are – all things being equal – better if low cartridge weight and higher muzzle velocity are more valuable for the purpose than projectile mass or swept volume. While we’ll discuss it more in depth later when it gets its own article, a high muzzle velocity is valuable if a flat trajectory and low aiming error are needed.

This is why we see relatively small bore and bullet diameters (for the size of the round) in most modern military small arms cartridges, as well as in long range precision rifle cartridges, and varminting rounds. In each case, a high muzzle velocity helps reduce the inaccuracy caused by elevation and windage errors made by the shooter. As well, in military rifle cartridges, ammunition weight is at a premium, so bore diameter is ideally kept to the minimum, so long as all necessary requirements are met.

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Most modern military rifle rounds have a relatively low projectile diameter to case capacity ratio, as they need to provide flat-shooting characteristics with low ammunition weight. However, in recent years there have been significant exceptions, like the .300 Blackout shown here, third from right.

 

In contrast, where bullet mass and swept volume are higher priorities (and, it is implied, where trajectory and ammunition weight are not), we see rounds with very fat, wide bores and large projectiles. In African big game rifles, which may at best only shoot a handful of rounds in an entire hunting trip, and which need high bullet mass and large bullet size to reliably tackle dangerous continental game, we see the opposite of what we get with long range precision rifles. Short, fat bullets coupled with only relatively modest case capacities (for the caliber) are the rule of the day.

Here we see the effect of increasing the bullet and bore diameter on performance. Below are two Powley sheets for the .260 Remington and .308 Winchester, both using the same weight bullet (123gr). Both bullets have been seated to give the same effective case capacity, so that the only difference between the two rounds from an ignition standpoint is their bore and bullet diameters. We see that the .308 Winchester produces 9.3% more muzzle energy and is three percentage points more efficient than the .260, even though both rounds have essentially the same propellant load and case capacity, and are fired from the same barrel lengths:

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Swept volume is valued for short barreled rifles, as well. Wider bores allow the propellant gases to expand more within a shorter length of barrel, allowing the gas to impart its energy to the projectile more completely within this short length. This is why the .300 Blackout and Russian 9x39mm calibers use such wide projectiles for their case capacities, and it’s why the former round can produce the same muzzle energy from a 10.5″ barrel as can a 5.56mm from a 14.5″ barrel, despite using only three-quarters as much propellant! As well, a greater swept volume to propellant mass ratio alleviates heat flux significantly, a subject covered more in-depth in another post I have written.

Next up, we take a look at the trade-offs involved in selecting different bullet weights. Stay tuned.

Disclaimer: I cannot within the span of an article of this size cover every single trade-off involved in each “axis” of design. My hope is that by the time the series is done, most of the major trade-offs will be covered. Please be patient with me 🙂



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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  • George

    …another implication of swept volume (more precisely expansion ratio) is lower gas pressure/temperature at the muzzle, reducing muzzle blast and making sound suppression easier. The significance of that is probably underestimated…

  • ColonelColt

    Thank you for writing these articles. These are some of the little nitpicking details of firearms design I’ve been trying to explain to people for years as to why one cartridge might be better than another. It’s not all about speed.

  • gordon

    I use .338 Spectre as my intermediate cartridge. For a short barrel, subsonic cartridge, it is the bees knees. I also like it because it looks like a mini version of the 13x64B and 20×82;-)

    • Knee Slider

      Fantastic Gordo

  • Paul Epstein

    I think this is a good reason to look further into sabot technology. Ballistics in the barrel overwhelmingly favors a large caliber bullet, ballistics outside the barrel overwhelmingly favor a smaller caliber for the weight. We’ve known that for a while. Sabots have a lot of weaknesses, but they bridge the gap between those two needs in a way that I can’t see any other tech doing.

    • roguetechie

      CONVENTIONAL SABOTS have a lot of weaknesses.

      However, the improvements in manufacturing technology we’re beginning to see actually show up in production environments as well as our abilities to use various computationally intensive simulation technologies are quickly making those old limitations easy to overcome!

      …There’s actually an existing small arms sabot design which should be easily scalable which alleviates many of the previous flaws with small arms sabots. It’s just a matter of getting the fudds in R&D and the buying public to set aside all the stuff they think they know that’s preventing any serious work in this direction from being carried out.

      Basically the tribe of the gun as they currently exist are an almost hopelessly superstitious and change hating lot.

      • GD Ajax

        Which is why are weapons are going to be worthless in the next few years

        • Uniform223
        • roguetechie

          Luckily for us, not everyone is giving up and just letting this happen.

          As usual, it’s up to us garage innovators and experimenters to do the research and bring the technology we need to fruition!

    • roguetechie

      CONVENTIONAL COMMERCIALLY AVAILABLE small arms sabots have a lot of weaknesses…

      Sabots aren’t the issue, it’s the sabots that have up to this point been available to shooters that are the issue.

      Two very different things…

      Luckily manufacturing technology, design software, ballistics software, and FEA software have now brought us to a point where we no longer have to accept these very flawed sabot designs anymore.

  • Max Müller

    While v0 (bullet speed at barrel) might be higher with the .308, the 6.5mm bullet will be better at range. It has a much better ballistic coefficient and thus will slow down less and has a higher velocity and thus more energy at say 500 or even more at 1000 yards. Plus, it will drop far less which makes aiming much easier.

    • Dr. Longfellow Buchenrad

      Correct me if Im wrong, but I thought 6.5CM has a higher v0 than .308. Though you may be speaking of Grendel which I do not know about.

      • Not with the same weight bullets, they don’t.

        • Dr. Longfellow Buchenrad

          Id say youre right about that, but I think the vast majority of 6.5s are 140gr or less while the vast majority of .30s are 147gr or more. At least the ones Ive seen.

          Im not trying to argue. I actually do want to learn and be corrected where Im wrong.

          • That’s correct. 6.5 CM generally uses lighter bullets than .308 fired at a higher muzzle velocity.

          • Dr. Longfellow Buchenrad

            Ive had a question for a while and I think Ill throw it at you since you appear to be the most qualified person I know to answer it (the answer may fit somewhere in one of your future articles in this series and if so, Ill patiently wait)

            How much does the amount of contact surface between the bullet and bore affect muzzle velocity? Like, with all other variables being equal, the difference between a Boat Tail Spitzer (a relatively small contact surface) and Non Boat Tail Flat Point of the same weight (a relatively large contact surface)?

          • The short answer to that question is that the longer contact surface will marginally decrease the efficiency of force transmission to the projectile from the gas due to frictional losses, decreasing its eventual net muzzle kinetic energy and velocity.

            But that’s not really a complete statement. The long answer is “this is very complicated and often this effect gets swallowed up by other differences.”

            I’ll leave you with something concrete to chew on: We see with M855, which has a relatively long bearing surface, that it has substantially increased friction versus M193, probably because of the longer bearing surface.

          • ostiariusalpha

            Of course, there are ways to play around a bit with how much friction you get besides just looking at the amount of area making contact. Since the projectile’s force against the bore is supplied by its material elasticity, you can have altered friction by changing between the various alloys of solid lead, solid copper, copper-jacketed lead, bimetal jackets, plastic sabots, the different frangible bullets, and whether or not it has a different core material (like a steel penetrator). For a given material, friction simply scales linearly with surface area making contact, so that does generally have an easily measurable effect on muzzle velocity.

    • Right, we’re just dealing with v0 here, we’ll get into retained energy later.

  • Dr. Longfellow Buchenrad

    Great articles. Great stuff to be learned in all of them.

    PS: Unless my life has been a lie up to this point, wouldnt the .300blk be the third from the right not the left in the above picture?

    • My mistake! In my defense, I had the flu when I wrote this.

      • Dr. Longfellow Buchenrad

        Its all good. If I wrote as many words as you do I probably would do it much more frequently.

  • SlowJoeCrow

    Can you add a ballistic chart to the Powley chart of .308 vs .260 since the velocity and bc effects of 6.5 vs 7.62 are significant.

  • GlennG

    Caption correction: 300 Blackout is third from the RIGHT.

  • noob

    If sabots worry you about accuracy and danger zone template, what would happen if the projectile was made of very strong and lightweight material to decouple bullet weight and swept volume?

    So a wide projectile made of carbon fibre wrapped around a steel penetrator and jacketed in copper that is electrochemically bonded to the carbon fibre body, endowed with an excellent BC, could achieve and retain a very high velocity from a shorter barrel with less propellant.

    Is there something i missed that scuttles this idea?

    • CommonSense23

      Price

    • Sounds like it would have very poor sectional density, and thus poor velocity.

      But I think the “problems” of sabots are overblown, anyway. Cup-type sabots would probably be fine for an infantry weapon.

      • iksnilol

        Dude, we don’t want our budgets slashed.
        -DARPA

    • roguetechie

      Aside from this approach being extraordinarily sub par compared to what Dr. Voss did at CETME, and the US government did with the FABRL round, it being freakishly expensive, and that the CBJ sabot even scaled up should have neither the danger zone or glass fiber particulate ingestion and eye danger that concept has many other potential problems.

      • PK

        That 6.5mm sabot does indeed scale just fine to other sizes and payloads.

        • roguetechie

          Do tell please PK!!!

          or at least hint me in the right direction… You have no idea how helpful it would be!

          I ****ing hate having to duplicate existing research if there’s a way to avoid it.

          • PK

            A decent description, and the current use of said sabot in things such as .300BLK, is available in the company literature along with all published test results so far.

          • roguetechie

            That really makes me curious about what a 7.62×38 FABRL design using a cbj sabot would accomplish.

            Along with many other now pressing questions. Also the PDF they now have on their site confirms quite a bit about why it works well and etc that squares pretty exactly with why I theorized it was worth pursuing and how it does what it does.

          • PK

            You’ll probably be able to figure out the exact thinking from the sectioned renderings. It’s a simple solution to the problem, and it works well. As soon as the sabot/core combination leaves the muzzle, the rear check comes off and things separate. Apparently the sabots all land fairly close to one another, as well.

          • roguetechie

            Oh yes that part I get and am pretty familiar with because I’ve had rotating pictures from the patent as desktop background for years….

            Hell, I actually got into 3d printing several years ago specifically so that when the materials and resolutions caught up to cbj sabot level I’d be able to hit the ground running lol.

            It’s been pretty much an obsession with me for years now.

          • Linky plz?

      • noob

        I’m interested in FABRL but i get no results searching for it. Where can I learn more?

  • Don Ward

    At the end of this series, I’m waiting for Nate to definitively prove that the 30-06 is the best round after all.

  • valorius

    I’ve invented a new caliber, i call it the .306 Trump. It’s a magnum, of course. It’s proven itself capable of utterly destroying ‘impenetrable blue walls.’

    • DW

      You developed this with a small loan of 5 million dollars right?

  • A Fascist Corgi

    Most of the combat videos that I watch on an almost daily basis show combat involving vehicles and taking place at long range. You typically can’t even see what they’re shooting at. The combat soldiers that I see also aren’t carrying 80+ pounds of gear. They’re typically just wearing their body armor and a small, trim combat pack (or no backpack at all) that’s about the same size as a kid’s school backpack. That’s why I think that you’re wrong for being overly obsessed with weight.

    Increased weight be damned, the modern soldier needs a magnified optic, a longer barrel, and a better long-range round in my opinion. The 5.56 round simply isn’t optimal for modern combat anymore in my opinion.

    You know that I’m a fan of NATO transitioning to the 6.5 Grendel round but that’s only because I’m not extremely knowledgeable about all of the other options that are out there. I was never entirely satisfied with the 6.5 Grendel round because it was being forced to work in a standard M4 rifle, which I think gimped its performance. I only picked the 6.5 Grendel round because it was the best round that I had personally heard of. That’s why I was wondering if you knew of a round that met the following criteria:

    Slightly longer and a bit more powerful (around 1,800 foot-pounds of energy) than the 5.56 round but still controllable under full auto fighter. It should also have the best ballistic coefficient possible in order to have the best long-range performance out of a standard-issue assault rifle. And while I’m not as concerned with weight as you are, picking a round that was as light as possible while still meeting the above criteria would be ideal.

    • crackedlenses

      As long as our battlegrounds are set in Afghanistan, you would probably be correct.

      • A Fascist Corgi

        Most of the combat footage from Iraq and Syria are also at long range and involve huge vehicle convoys.

        • crackedlenses

          Yes, because most infantry combat now and into the foreseeable future takes the form of defending massive convoys and shooting at targets across mountains. Why not just issue everyone sniper rifles or LMGs and be done with it?

          • A Fascist Corgi

            You act as if putting a magnified optic on a bullpup rifle with a 20-inch barrel and a better round is completely impractical and absurd, or somehow detrimental to other forms of warfare.

            Why is everyone so obsessed with defending the M4 and the 5.56 round? You guys act like the U.S. military stumbled upon assault rifle perfection when they adopted the M4 and so there’s no point in improving upon something that’s already flawless.

          • User

            Nobody says the 5.56×45 is absolutly flawless…
            But 6.5 Grendel is just far from what is sencefull to adopt for the many reasons ive written.

            Metal bottleneck infantery cartridges like i said dont have much if any chance for wide NATO adoption. But rather polymer cased (bottleneck, or cylinrical, cylindrical is more modern and has mechanism advantages).

            And than still a 6.5 (6,7mm) diameter is just too wide to be usefull and give high performance in an Infantery Rifle.

            Adopting 6.5 Grendel is absolutly senceless for the many reasons ive listed. And as said serval times better cartridges are on the way. Ofcourse 5.56×45 will be replaced, but some since decades outdated metal bottleneck cartridge isnt nearly worth the immense cost at all. That rather will be invested when its time. As said its coming.

          • crackedlenses

            “You act as if putting a magnified optic on a bullpup rifle with a
            20-inch barrel and a better round is completely impractical and absurd”

            Well, let’s see, we’ve got to retrain everyone to use bulpups, pick a bulpup (that’ll go well, sure), adopt a completely new rifle round and LMG and/or H-BAR rifle to accompany said bulpup, expand training to enable our riflemen to hit targets out at 600, 700, 800 yards….

            Rather impractical, yes. Absurd? Certainly if you don’t think we need infantrymen to shoot past 500 yards.

            “and so there’s no point in improving upon something that’s already flawless.”

            Find me something worth the price of replacing the M4 and 5.56 mm. round and then we’ll talk. The Army to date has not. They are developing LSAT, and when that becomes viable perhaps we’ll see a new round.

          • n0truscotsman

            …because the M4 and 5.56 are the most economical options currently. You get modern modularity and effectiveness at a relatively inexpensive price.

            As soon as something else comes along, which is more economical, then the M4 will fall to the wayside. On the account of the proliferation of 5.56 worldwide, it would have to be something truly game changing.

            The adoption of another cartridge-fed rifle, which is essentially a 100 year old plus technological concept, will simply be running into the ‘diminishing returns’ problem.

    • The 6.5mm Super-Z’s stated numbers give muzzle energies higher than 1,500 ft-lbs, but I think they are a little optimistic. If you used what I would consider to be more reasonable figures, it would be closer to what you are looking for.

      Alternately, you might take a look at the various 6.5mm/.223 wildcats that have been developed over the years. Loaded to longer OALs, they would meet your criteria easily. Here’s an example, the 6.5mm MUTT, developed by Bevan over at SnipersHide.

      • A Fascist Corgi

        The Super Z is pretty close to what I had in mind, except that it’s a bit too heavy and still a little bit too short. I’m starting to agree with you that a light weight, high velocity round is better. But I’m still not satisfied with the 5.56. If that Super Z bullet weighed around 100 grains and had a velocity of around 2,800 feet per second and still had a good ballistic coefficient, then it would be an ideal assault rifle round and a worthy replacement for the 5.56 round.

    • Uniform223

      “Most of the combat videos that I watch on an almost daily basis show combat involving vehicles and taking place at long range.”

      > Speaking from experience I see…

      “Increased weight be damned, the modern soldier needs a magnified optic, a longer barrel, and a better long-range round in my opinion ”

      > yup, definitely speaking as an SME with experience…

      “You know that I’m a fan of NATO transitioning to the 6.5 Grendel round but that’s only because I’m not extremely knowledgeable about all of the other options that are out there.”

      > Why advocate it if you’re not completely knowledgeable of the subject or what is out there? Are you speaking from “experience” from all your hours watching combat footage on youtube?

      “And while I’m not as concerned with weight as you are”

      > Unless you have ever had to carry that weight and work in it your opinion that “weight isn’t a concern” is male bovine fecal matter.

      • iksnilol

        Is this what you call an armchair commando? 😛

  • noob

    Thanks – That’s interesting! found the links. I was googling for “FARBL Round” but it kept correcting to “fabric round”.

    • roguetechie

      No problem lol Google fu is like tai chi repetitive, deceptively simplistic, and oddly powerful.

  • zipper

    Your comparison of the .260Rem to the .308Win is somewhat flawed. First, I’ve never encountered a commercially-loaded .308Win round with a 123gr. bullet(goes with the 7.62×39). Being primarily a big-game cartridge, the usual bullet wt. is typically 150-180gr. Having handloaded the .308 in the past, I’ve found the 165gr. to be the ideal choice for a BA hunting rifle. Military-type guns are best with the 147/150gr, which they were designed for.
    Next, while you’ve strived to equilibrate the two cartridges as much as possible, you used different powders. That is a huge discrepancy! Again, being a handloader, I can attest to the effects that using different powders can have, at least in the same cartridge, which is the Only basis for a legitimate comparison.
    Third, thank you for introducing me to the term “swept volume,” as I’ve never encountered it in any discussion on ballistics. The probable reason for this is that it’s inherently understood that a larger diameter bullet will naturally create a larger internal volume, given a fixed barrel length.
    It’s like saying a 427ci motor has a larger “swept volume” compared to a 255ci motor. Duh.
    Ballistics is a complicated business, with many interrelated factors in the mix.

    Your discussion only lightly touched on the reasons for choosing one cartridge over another. Since you went off-course bringing so-called “African” cartridges into the talk, I’ll add that with Magnum cartridges, you achieve the advantages of flatter-shooting, higher-velocity, larger, & heavier projectiles. But, the subject of your article was Intermediate cartridges, wasn’t it? That category can be further defined as those rounds that are intermediate in power, and those that are intermediate in OAL & other case specs.
    The major point, which you alluded to, but never really discussed, is the Application. Military use, competition, or small game hunting? Although there are some overlapping factors, each category places an emphasis on different aspects of a given cartridge’s performance.
    Much has to do with the preferred bullet choice for each respective application. E.G.- For the same cartridge, one might choose a higher velocity load to ensure positive functioning of expanding bullets for hunting. But, for target competition, that load might be reduced to a level providing optimum accuracy. Of course, bullet weights and type will follow each application.
    All that said, I commend you for attempting to address the the differences in these cartridges, based on ballistic considerations.