Weekly DTIC: The 5.56mm FABRL

556x39FabrlG

Today’s DTIC document – from June 1973 – covers the early development of the 5.56mm FABRL cartridge, which is one of my favorites. For those unfamiliar with it, the 5.56mm FABRL (Frankford Arsenal and Ballistics Research Laboratory, later rationalized to “Future Ammunition for Burst Rifle Launch”) was a cartridge from the early 1970s intended to reduce the recoil and weight of current and projected rifle designs, allowing for more controllable and higher cyclic rate bursts, as well as lighter ammunition. Key to this principle was the extremely low form factor bullet (lower form factors are better, and the AR-2 bullet had a smoking form factor of .84 i7, compared to M193’s 1.24) which allowed the very light 37 gr .22″ caliber bullet to maintain the same ballistic coefficient as M193, and the same trajectory when fired at the same velocity. Further, the bullet was flat-based, improving barrel life, and in order to reach 3,270 ft/s, only 39,500 PSI chamber pressure was required from the shrotened 1.5″ long case, which opened the door to lightweight aluminum cased ammunition. This resulted in an extremely lightweight round, weighing in aluminum alloy cased form about half that of M193, with the exact same external ballistics. It’s not difficult to see why this was an attractive alternative to other competing future ammunition projects.

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A lineup of experimental small caliber cartridges. Aluminum cased 5.56×38 FABRL is second from right. Image source, quarryhs.co.uk.

The round itself featured a spindly flat-based bullet with an extremely long ogive, a steel penetrator (not unlike the later M855 round) and a plastic core. This projectile configuration would later be echoed in the SS90 round of the early production P90 submachine gun. The 5.56x38mm case was the same as 5.56mm, but shortened to just over 1.5″ long, and was made from both brass and aluminum alloy.

In my opinion the 5.56×38 FABRL represented the concept with the most potential relative to its ambitiousness. Indeed, the Russians would follow the same route during this period, choosing a low form factor bullet for their 5.45×39 round, with a similarly short case. A Limited Analysis of a New Ammunition Concept for Potential Future Rifle Application is a pretty short read, and has something for anyone with even a passing interest in experimental 20th century small arms.



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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  • Zachary marrs

    I wonder what this weeks argument will be over ;p

  • kev

    It looks rather like a miniature 408 cheytac

  • phale

    Why didn’t it catch on, then?

    • Frankford Arsenal was shut down. More specific than that, I don’t know.

      • I can’t imagine that the pending NATO standardization trials helped any in trying to justify the funding for any new cartridge types.

  • TheRealDan

    looks like 5.45×39

  • ColaBox

    Second from the left, what in the world….

    • Dan

      That was designed during the height of the grasshopper invasion thankfully we didn’t need it as diplomacy won out in the end.

    • .12″/3mm US experimental, from testing micro-bore rifles.

  • Jeff

    Some of the projectile designs in the research document are intriguing. Plastic jackets, DE cores etc.

  • DashVT

    Anyone know the cartridge second from the left? I’ve never seen such a large cases necked down to such a small projectile.

  • big daddy

    I think the USA & NATO should dump the 5.56mm round and go with something else. They can than dump all that ammo cheap for the US market……dream on, right I can dream. I think the next revolution will be in ammo, it has to be. Unless they bypass that and make viable lasers for the infantry.

    • That would be a large logistical endeavor, and most of the alternatives available now don’t really seem like improvements to me.

      I think military small arms is fundamentally a field riding on the back of production engineering and logistics. Once those improve, and allow more complex, expensive, and better equipment to reach every fighting man in the army, those concepts will obsolesce what we currently have.

      Until then, though, what we have remains “state of the art”.

      • raz-0

        Compared to the level of success with the cased telescoping ammo, this sucker looks like a winner in comparison. The functioning versions of that stuff are at a 40% weight reduction or so. This looks like it beats that and is also more compact.

        On the other hand switching to it wouldn’t satisfy the hard on they seem to have for denying any of their inventory to make it to civilian surplus channels.

        • I’m not sure how much success they had with the aluminum cased version at the time, but today people at Aberdeen and elsewhere are still seriously looking at aluminum cartridge cases as a way to reduce weight, so maybe we could see something like this.

        • Rusty Shackleford

          The telescoped-cased ammo and LSAT LMG have come a long way over the last decade and seems pretty close to entering service, probably in 5 years or so. Caseless will definitely take more time and money.

          • I suspect we won’t actually end up with telescoped ammunition, but that the research from that concept will pave the way for polymer cased ammunition of a more conventional variety.

  • guest

    So it is the 5.45×39 that never was. Too late.