Modern Historical Intermediate Calibers 011: The 5.56×38 FABRL

An interesting concepts that was tested in the mid-late 20th Century is that of an extremely light for caliber, very long bullet made of lightweight materials like aluminum and plastic. The 7.92×40 CETME, which if I can find a specimen I will cover later on, is one example, but starting in 1972 the now-closed Frankford Arsenal began experiments on 5.56mm cases loaded with super-long projectiles with von Karman ogives, with the aim of creating a lightweight round using a low-density projectile and an aluminum case. Original testing was conducted with full-length 5.56mm cases and two lengths of bullets, but eventually a shortened brass case and a 37gr bullet with the same shape as the shorter initial test bullet was created. As a solution to the problem of burn-though with aluminum cases, the Arsenal developed a plastic insert called a “flexible internal element” (FIE), and the brass cased rounds developed for ballistic testing also had FIEs. This shorter round in both aluminum and brass cased forms was called the 5.56x38mm FABRL, which stood for “Frankford Arsenal – Ballistics Research Laboratory”, and this at some point was made into a backronym for “Future Ammunition for Burst Rifle Launch”.

The 5.56mm FABRL’s projectile, due to its shape, has an exceptionally low form factor (0.836 i7), and a resultingly high ballistic coefficient for its weight (0.126 G7). Using G7 for the flat-based AR-2 Short projectile is strange, and I normally would convert into a drag model for flat-based bullets, however I actually have empirical G7 numbers for this projectile from the Aberdeen Proving Grounds, and I do not want to change this data artificially.

One important thing to note is that although I show data for the 5.56mm FABRL out of a 14.5″ length barrel, the cartridge predated the M4 by over a decade and was never tested from barrels of that length.


Ballistically, the 5.56x38mm FABRL essentially equals the 5.56mm M193 in every way except energy, which is about 35% lower at the muzzle, but where this round really shines is in weight. The brass-cased 5.56x38mm round weighs in at about 9.7 grams per shot, and while I don’t have an example of the (very rare) aluminum cased round, I estimate that it weighs about 5.8 grams per shot. That is an enormous 50% reduction in weight versus the M193 round!

Note: All ballistic calculations are done with JBM’s Trajectory calculator, using the ballistic coefficient appropriate to the projectile being modeled, and assuming an AR-15 as a firing platform. Also, keep in mind that there is no single true velocity for a given round; velocity can vary due to a large number of factors, including ambient temperature and chamber dimensions. Instead, I try to use nominal velocity figures that are representative of the capability of the round in question.

Postscript: Since I’ve decided to expand this series to calibers that – while they may still be ballistically relevant to the wider conversation about intermediate calibers – are no longer actually being produced or tested, I have decided to re-name some of the posts in this series to “Historical Intermediate Calibers”. However, I will retain the same numerical sequence and the word “Modern” struck through. Enjoy!

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 is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at


  • ostiariusalpha

    Plastic bullets instead of plastic casing, somebody had to try it, I guess. Now for a truly bold experiment where both the projectile and case are polymer!

    • Sermon 7.62

      There’a a bolder experiment: caseless ammo.

    • Gary Kirk


      • ostiariusalpha

        Air… HARD!! Heh, I’m mostly kidding about the plastic bullets, it’s just the FABRL bullet is so light it might as well be a copper coated polymer projectile. Like Anonymoose mentioned, it’s kind of hard to believe it would have adequate penetration with that light of a bullet.

  • Wolfgar

    The shape of the cartridge and case look’s suspiciously like the 5.45X39.

    • yodamiles

      Yeah, similar idea. Shorter/fatter case to allow for longer bullet while maintaining powder quantity.

      • gunsandrockets

        Not exactly.

        Powder capacity was sacrificed, because there is a trade off of less case capacity in exchange for a longer bullet.

        • ostiariusalpha

          Yeah, the case head of the FABRL is the same diameter as the 5.56 NATO.

          • yodamiles

            You guys are right, since the round is so light (37g) it doesn’t require that much powder to push it to 3200 fps

          • Remember that these were being fired from 20″ barrels, too, so it really was a pretty sedate load, less powerful even than many .221 Fireball loads. But that was by design, as remember that the 5.56×38 was designed from the outset to use aluminum cases with plastic inserts to prevent burn-through.

  • yodamiles

    If we combine this bullet design with LSAT telescopic case, would it allow us to build even long and more ballistic coefficient round without worrying about overall length. Give that round a boat tail and you will have super lightweight low drag tactical fighting munition.
    11/10 would operate.

    • yodamiles

      Seriously though, I really want to know what Nathaniel F thinks of this.

      • If you look at the latest LSAT/CTSAS ammunition, it does not have any propellant forward of the bullet cannelure, which means that a long-ogive bullet like the AR-2 would necessarily have a poorer ratio of case capacity to cartridge weight than a more conventional shape with a boattail.

        Having said that, the general concept of a lightweight, low-drag round of .22 caliber or below is very promising, especially in a telescoped format.

        Plus, here’s another idea: The downside of a long ogive like the AR-2’s in a telescoped round is that now you need a longer and therefore heavier front plug, right? Well, what if you made that plug work for you by making it a discarding sabot instead? You would dramatically improve the swept volume in the barrel, giving you better performance for a shorter barrel length, and you would be facilitating full-caliber subsonic alternate loads.

        • ostiariusalpha

          That said, there’s no real reason why the bullet can’t be telescoped into the propellant with an LSAT type case. It’s a solid molded propellant, so there is very possibly more capability there than is being taken advantage of in the current LSAT/CTSAS design.

          • Except that if you do that your barrel wear goes waaaaay up and your propellant use efficiency goes way down.

          • ostiariusalpha

            Do you have a source for that? I wonder how much effect it really has.

          • This is me you’re talking about here, os. 😉

            Of course I have a source:

            You might say “well, that’s a pretty old source”, but it’s worth noting that LSAT tried that configuration and then switched to the one they’ve got now. I even remember one presentation which I can’t find right now that mentioned the barrel wear issue.

            “As a result of the higher propellant loads in cased telescoped ammunition, a significantly higher temperature and heat input is imparted to the gun barrel and barrel life is significantly reduced. To mitigate the deleterious effects of cased telescoped ammunition on barrel life, cartridge designs have incorporated an erosion inhibitor. The erosion inhibitor reduces the heat generated by the burning propellant and heat input to the barrel. The erosion inhibitor also reduces muzzle velocity of the cartridge as compared to a cased telescoped cartridge with no erosion inhibitor. Therefore, the inherent ballistic inefficiency of a cased telescoped cartridge is further magnified by incorporating erosion inhibitors. The incorporation of the erosion inhibitor within the cartridge also raises ammunition costs.”

            “For the cased telescoped concept to work properly, the projectile must move from the cartridge to the forcing cone of the barrel in almost perfect alignment with the axis of the barrel in order to obturate the barrel. In all research and development programs to date, obturation of the barrel with the projectile has been a problem and blowby of propellant gases has occurred. The resultant ballistic inefficiency prevents consideration of fielding such a system.”

            “The higher propellant charge of the cased telescoped ammunition produces higher temperatures and heat input to the barrel. These conditions have reduced barrel life of conventional barrel material to an unrealistically low level. Barrel life is reduced mainly by melting and erosion. Whereas the minimum replacement schedule of a conventional gun is approximately 10,000 to 15,000 rounds, cased telescoped ammunition guns with state-of-the-art barrel technology yield a barrel life of about 200 rounds. The difference in barrel life between a conventional and cased telescoped ammunition gun is two. orders of magnitude.”

            That doc is full of stuff like that.

          • ostiariusalpha

            I knew you must have something, though I was hoping it would be more up to date concerning the actual propellant and internal case geometry of the current 5.56mm CTSAS. I’ve read the 96 report before and was familiar with their findings at that time. Basically, I’m most interested in the performance and erosion gradients; if you telescope the propellant up to or past the bullet’s meplat, that might give undesirable results, but what about 3/4 of the way along the ogive? Halfway? A fourth? Where is the point at which the LSAT/CTSAS propellant becomes noticeably more detrimental than helpful? A good graph of something like that would make everything a lot clearer.

          • Oh, I’ve got no idea. I would try asking Kori Phillips. 🙂

          • ostiariusalpha

            Ha ha, I’m sure she is a knowledgeable resource, but I’d prefer a different perspective. Perhaps one that doesn’t seem like it is trying to play booster for a 6.5 caliber cartridge. ?

          • I haven’t spoken with her yet, but I half suspect that when I will she’ll be a lot less warm on the subject.

            The thing is, when Benning sends Picatinny a requirement, Picatinny goes “yes sir yes sir three bags full”.

          • ostiariusalpha

            Here’s hoping you get to have an illuminating conversation with her.

          • yodamiles

            So, did HK G11 caseless ammo had the same issue with propellant forward of the bullet.

          • Yes, that is my understanding. It had a lot of unsolved issues.

          • George

            The CT cannon rounds that go back further tend to do a stronger tube-in-tube and run the projectile further back, but you hit minimum gauge limits etc. Small arms are not mini cannon shells…

        • yodamiles

          Interesting, I thought that one of the biggest advantage of telescopic case is to have any propellant forward of the bullet cannelure. Nonetheless, I was kinda disappointed to hear that development of 5.56 LSAT round was ceased to focused on its big Cal brothers.(based on your article in May this year)

          • Yeah, it turns out that if you seat propellant ahead of the bullet, it burns before the bullet can move in front of it and chews the throat up. So they solved that problem by just making the plug larger.

        • George

          The swept volume in barrel argument is usually skipped right past.

          There is much unneeded allergy to sabots but that should be overcome. Some issues with suppressors, however- base type sabots may be necessary.

        • Giolli Joker

          Basically you’re proposing sort of an high pressure, rimless, shotgun round with sabot slug?

          • …Not really, hahah.

          • Giolli Joker

            However, more seriously, if you think about it, a CTA, if you remove the detail of the projectile base being below the powder level and you reduce the difference in OD between case and bullet, is not that far from more conventional designs as shotgun shells.

  • Vitor Roma

    One thing that the article forgot to mention about this caliber is how low pressure it is, something around 39,5k psi, basically a hot pistol cartridge pressure. Given the low weight and the low pressure, it would have minimal recoil and also privde extreme longevity for the gun.

    • gunsandrockets

      Hmm… low pressure and a short case allows more options for practical breech design too.

  • Anonymoose

    37gr? I think we’re going in the wrong direction here. Could it even poke through a Soviet steel helmet?

    • ostiariusalpha

      At 3250 ft/s? Probably not. But, if you bump the speed up to 3600-3700 ft/s, I bet that thumb tack could do some pretty impressive stuff.

    • Yes, I bet it could, although out to what distance I don’t know. Probably at least 300m.

    • Gary Kirk

      With proper construction.. Could be a nasty little S.O.B. bet with that long a bullet they’d fragment with even a slight yaw in target, and at pretty low velocity

      • Bullet construction is a nylon core, steel penetrator, and gilding metal jacket.

        • Giolli Joker

          Is a cross section of the bullet available?

    • Don’t forget that the original 5.7x28mm SS90 projectile used a 23gr polymer core projectile. The current SS190 projectile is only 31gr. So the FABRL projectile should be able to out perform either of these.

  • gunsandrockets

    Could a similar projectile scaled up fit the .300 AAC? Maybe.

    But in the meantime there is the 125 grain Sierra Matchking which is unusually pointy for its weight.

  • Hey guys, since some of you asked, I wanted to give you some idea of how well (relatively) this round retains velocity. A comparable bullet of normal design is the Hornady 40gr V-Max which is launched from the P90 at about 1,950 ft/s. That load is regarded pretty well for its antipersonnel capabilities, so let’s see how it compares from a 5.56 FABRL launched from the same 10.5″ length barrel:

    • What this makes me want, really, is an AR-15 chambered in 5.56×38, firing scaled down 40gr Barnes .300 Blackout TSXs with the hilariously large tip and body cavity. All of the expansion range, and no recoil whatsoever, yippee!

      • Fox Hunter

        Better yet, replace all 556×45 with this, you could probably shoot 556×38 in a stadard 556×45 ar15 with zero changes.

        • That is one of those things that you technically can do, but really, really do not want to do.

  • Fox Hunter

    this would have been a great replacement for 556×45

  • mechamaster

    Look’s fun for the bullpup 5,56×38 assault rifle.
    ( even the bullpup is unpopular nowadays )