At the end of last year, we told you about the US Army Marksmanship Unit’s .264 USA round, which has been designed for an intermediate length AR platform rifle larger than an AR-15 but smaller than an AR-10. The round is one of the first to incorporate polymer case composite construction so early on in its development

Tony Williams, co-author of the book Assault Rifle with Maxim Popenker, sends along this image of the .264 USA cartridge in both conventional brass-cased and composite brass-polymer case guises:



Alex and Patrick of TFBTV recently related their story shooting composite cased PCP ammunition, and the resultant explosion that could have seriously injured one or both of them. Polymer composite cased ammunition is an ambitious idea, and PCP’s incarnation of that concept is seriously unsafe. Not only has the .264 USA possibly been designed specifically for composite cased ammunition, or at least with it in mind, but the composite cased ammunition itself is not being made by the PCP ammunition company, but rather a company called MAC, LLC, out of Mississippi. One of MAC, LLC’s relevant patents is available here.



Readily apparent from the patent image is the greatly increased thickness of the case wall for the MAC, LLC case design. This reduces the capacity of the case, but is necessary given the use of polymer material in the place of brass.


Additional information about MAC, LLC’s .50 caliber polymer cased project can be gleaned from this presentation from 2012.


  • HMSLion

    This could get interesting. A caliber switch would be the one thing that could lead to the replacement of the M-16 platform.

    • I don’t think any caliber change will precede the development of a new kind of ammunition. I also have a lot of skepticism about the universal ammunition concept.

      • Anon. E Maus

        Yeah, I’m not buying it.
        You’ll never have a one cartridge fit for all purposes.

      • Ives67

        I think theres a big missconception that a true intermediate caliber would supplant the use of different calibers within the military. While something like a .264 round would better handle multi uses for the average unit, doing the job of general issue but also unit sniper duties. It seems that it would allow for purpose built cartridges like 300 win. mag, or 338 Lapua or Norma to take up a larger role.

        • The .264 USA would add a lot of weight and recoil vs. 5.56mm, so I think while it might be able to replace 7.62mm in some convenient roles, you would still need something smaller for the general issue carbines.

          • CommonSense23

            One of the things you got to consider with anything that comes from the AMU is that a lot of what they propose has absolutely no combat use what so ever. A lot of their guys have never served in a combat role what so ever. A lot of the things they propose are great for competition but that’s about it.

      • Riot

        I’ve always saw it as a bump up rather than a universal round.
        Having a much better replacement for 7.62 infantry rifles could allow the introduction of a longer ranged, more specialised MG and sharpshooter cartridge.

      • Monty01

        The concept of an intermediate round to sit between 5.56 mm NATO and 7.62 mm NATO was always a flawed concept. The fact is that so many regular armies, insurgent / terrorist groups and criminal gangs use large calibre rounds such as 7.62 mm NATO and 7.62 x 54R, 7.92 mm, .303, .30-06 and so on, that any army that adopts any caliber below 7.62 x 51 mm NATO will be overmatched. This is what happened in Afghanistan. A significant number of Taliban groups were using 7.62 mm NATO G3 and MG3 weapons made in either Pakistan or Iran.

        So if you’re going to adopt any new caliber it has to be better than 7.62 x 51 mm NATO. This is exactly what US AMU’s new is – a better performing round than 7.62 mm NATO in a lighter package. That is a significant step forward in my book. Forget the packaging of the round, even though MAC’s design is incredibly clever and effective, the basic ballistics of .264 / 6.5 mmm USA are superb, game-changing specifications. Ignoring the politics of changing calibers, there is no reason why .264 could not substitute 7.62 mm across all current applications and not be better.

        Where does this leave 5.56 mm? This is a PDW calibre for short-range personal defence. Period.

  • Cornelius Carroll

    Perhaps it just makes more sense to go straight to caseless. A caseless intermediate round fired out of a modern bullpup could replace the 5.56 and 7.62 with very few negative tradeoffs.

    • Caseless is not mature; and it’s not clear if it ever will be. The current focus is on lightweight case designs.

    • Javon McCabe

      The main issue with caseless is keeping rounds from cooking off. Once stable propellants are developed then the sky is the limit.

      • The propellants also have to be durable, energetic, stable in high and low humidity, consistent-burning, readily and consistently fragmented by an initiator charge…

        Quite a few engineering challenges, actually.

        • Canadian Vet

          There’s already a number of insensitive propellants and explosives out there. One that comes to mind is the one used in rounds for the Bofors 57mm gun that is in service with the Canadian Navy. Granted, that gun is electrically fired, as opposed to using a firing pin but under impact or severe heat the rounds won’t go off. Granted, that particular mixture is optimized for what is in effect a light artillery piece and while I won’t pretend to be an expert it strikes me as being a good place to start for a small arms application. Not to mention the design and R&D work involved with scaling down the ignition and trigger mechanisms to make them fit in a rifle instead of the breech of a naval gun.

          • The rate of fire of that artillery piece is also quite a bit lower.

          • Canadian Vet

            That is a very good point. The Bofors 57mm Mk2 is considered a rapid-fire gun with it’s rate of 220 rounds per minute or so and it is largely dictated by the feeding system and the size of the rounds and that rate is pathetic compared to modern small arms, and if heat is an issue there is a whole lot less steel and surface area to absorb and/or radiate excess heat in a rifle.

            But with insensitive electrically-fired propellants, the heat issues are more likely to have something to do with mechanical failure due to metal expanding at different rates based on the size and mass if each part (ie: the Ross rifle from WW1 which would seize during extended periods of rapid fire, or the Dreyse needlefire which would fail to eject under similar conditions).

            As for the firing system, there are already electronic triggers out there so we know it is possible to cram the required circuitry in there. Adapting the Bofors system for small arms is, therefore, probably possible but I’m not certain if it could be shoehorned, say, in an AR or SAW type platform but would most likely require a whole new platform to be wrapped around the ignition system.

      • JoelC

        It’s not just the cookoff. The heat itself becomes a huge problem. There is a way to fix it, but you need a high heat transfer material like manufactured diamonds. Once manufacturing techniques and material catch up, ceaseless ammunition will be rediscovered (provided metallic bullets don’t become obsolete first.)

    • Bal256

      Caseless currently has way too many disadvantages which make it unsuitable for rough military handling. But it sounds futuristic so people keep suggesting we adopt it.

    • Zachary marrs

      Very few negative tradeoffs?


  • Tet

    Are polymer cases re-loadable?

    • I think it depends on the exact configuration… But if you got inventive I’m sure you could reload them.

      • ostiariusalpha

        Even if the polymer section isn’t reusable, if they make it so you can dissolve whatever keeps the plastic from unscrewing out of the brass, you can install a replacement “hull” in the base and still save money. And even time, since there’s less resizing and no trimming needed.

        • Southpaw89

          I wouldn’t be completely surprised if the polymer is reusable, so long as it can handle the heat. I don’t know much about polymers but I would think it might handle more firings than a brass case, possibly being less prone to stretching and fatigue than brass.

        • I doubt they will make the polymer hull dissovable in anything common if they can help it – military ammo has to be solvent resistant, after all.

          • Secundius

            @ Nathaniel F.

            What about wrapping the Polymer Casing in 3M 433 High-Temperature Aluminum Foil Tape, which is rated at +600(deg)F…

    • Nero

      Yes the patent showed that. Threaded primer base. New plastic insert threads in

      • Anonymous

        Doesn’t look like threading, it looks like it’s non-removable.

        • Yes, those are grooves, not threads.

        • wzrd1

          Expansion of the brass base would cause deformation, compression would likely create a thread result out of tolerance to permit the plastic to thread the new dimensions.

      • Southpaw89

        If that’s so it could open up a lot of possibilities, with one set of parent bases you could reload for multiple cartridges, I expect the inserts would be pretty cheap, also if these are successful I cant help but wonder how long it would be before reloading companies started offering a means to form your own inserts at home.

  • Kev

    As far as I know MAC LLC 50bmg is called the mk 383 and is being tested as of 2015, I’m not sure if they are related but it’s made by another company possibly for manufacturing ebr inc. I also hear that MAC LLC are also developing a 300 win mag version. I belive a list of presentations was posted on the firearms blog some time last month, they had presentation s with people from ARDEC and pacatinny arsenal and even Anthony Williams. Some fascinating stuff on a new intermediate cartridge.

    • Yup, all that’s in the 2012 presentation.

      Tony speaks at NDIA every year, actually.

      • Kev

        Very interesting stuff. Thanks for keeping us posted.

  • Esh325

    I don’t really think a universal cartridge similar to this round has really ever been fielded and tested in combat, so it’s difficult to say about the utility of such a cartridge. I honestly don’t think there’s going to be new ammo adopted until something radically different like LSAT becomes viable. Imo, the 7.62×51 was probably the biggest mistake to adopt. A 6-7mm would have been a better choice for a marksman and machine gun cartridge. The Russians tried to replace the 7.62x54R with a 6x49mm which was quite high velocity, but it was canned due to the break of the USSR and economic problems. They had the right idea with that.

    • With the benefit of hindsight, I think the best round in the crop of NATO cartridges tested in the late ’40s and early ’50s was probably the 7mm Compromise (very, very similar to the later commercial 7mm-08 round), but at the end of the day, 7.62x51mm was not a bad choice, even if there were other options that were clearly (but marginally) better.

  • Swarf

    So a polymer case screws in to a brass cup?

    I don’t follow ammunition trends like some of you, so maybe I’m missing something, but that seems like a lot of manufacturing steps to add. Wouldn’t that make this round prohibitively expensive?

    • ostiariusalpha

      Nope. Brass is super easy to cut threads into, and the plastic threads pop out of the mold ready-to-go. The multiple steps required to form the length & neck of a full brass case are much more time & labor intensive for the case making machines.

      • Swarf

        Hmm, yeah, I guess I can see that, thanks.

    • It’s not threaded. Those are locking grooves.

  • Southpaw89

    Just by appearance these look a lot more durable than the PCP ammo.

  • Darkpr0

    Thanks for the post, Nathaniel. I didn’t even realize they were pursuing a polymer version of this case. We can see a couple interesting things…

    It’s not a totally polymer case which isn’t terribly surprising. Polymer won’t like the violent extraction that frequently comes with military firearms (particularly stuff like the G3) and so it makes a lot of sense to keep the rear section of the case as brass if you want this to work in a conventional firearm. It will eat away at the advantage of using polymer as a weight saving material but I guess that’s the price you pay for trying to sell to government organizations who have more or less shown that the last thing they want is firearms that are different in any way to what they’ve used before.

    The polymer is NOT threaded into the brass. The drawing shows ribbing, and if you read into the patent you’ll arrive at this: ” the caselet portion 12 a and the cap portion 12 b are held together by means of ribs 16 on the outside surface of an extension 18 on the caselet 12 a and ribs 20 on the inside surface of the cap 12 b which snap fit together when the cap is press fit onto the caselet.” which tells us that it will be difficult to remove these two. Threads are not as suitable for this because they would leave a helical pathway for gas to exit outside the gas seal. The threat then would be that pressure occurs between the casing and the chamber which would be disastrous. Ribbing will not allow this because it will tend to self-seal. There are threads which can provide a seal, but an interface between brass and polymer that will seal under these circumstances would be a pretty difficult task.

    Those ribs are HARD to make cheaply. You have a couple options: You can form them while drawing the brass which will require a mold which self-collapses which requires a ton of setup cost and is still probably difficult as hell. You can also machine the ribs pretty easily, however this requires an extra manufacturing step where you take each case and put it in some sort of lathe. It’ll do a great job with those rings, but it requires more time and effort so you end up with the same not-cheap thing. Again, price you pay for making the ammo work with conventional rifle systems.

    Also I’m pretty sceptical of intermediate rounds in general. I think there is a good argument to replace 5.56 OR replace 7.62, but you won’t get satisfactory performance out of one round. You think there are internet armchair generals complaining about the performance of 5.56? Wait until you try to replace both rounds with one.

    • Kivaari

      I think the ribs could be made fast by a collapsing die. Too hard to explain here for me other than it would appear like a collet die.

      • Darkpr0

        Concepts are easy, but the devil is in the details. Collapsing things are always difficult because the parts need somewhere to go. If you want to put a feature on the inside of a case you are heavily restricted in where things can go. It is not a trivial problem.

      • wzrd1

        I was thinking the same thing.

        Likely, they are.
        Knock off manufacturers probably won’t, with cartridge failure common in the knockoffs.

  • Tassiebush

    In the photo the partly polymer round seems to have a brass neck and shoulder insert as well as the casehead and an amount of powder seems to have squeezed in between this and the polymer section. That couldn’t be good. It would have to affect accuracy and probably safety.

    • Hi Tass, that’s actually just the shank of the bullet you’re seeing distorted through the transparent polymer.

      • Tassiebush

        Oh I see what you mean.

  • E.D.M.

    I, for one, am extremely intrigued by the idea of an intermediate sized AR platform. That opens up a whole list of possibilities.

  • Secundius

    The Problem is that PCP Cartridges are rated at +425(deg)F. When the Barrel exceed that temperature, the cases start to MELT. Above +500(deg)F, your going start getting Cartridges COOKING OFF inside the Barrels…

  • LG

    Remember, the metal case in a conventional weapon is also a major component in a weapon’s heat dissipation. The brass, or other metal, case removes a substantial amount of heat from the weapon. Just look at the differences in barrel life of the 243 Winchester vs. the 6 mm Remington. The slightly longer neck of the 6mm Remington dissipates and removes a significant heat burden in the weapon with a palpable prolongation of barrel life.

    • Paladin

      A polymer case might actually help a firearm run cooler. Polymer does not conduct heat nearly as well as metal, so where heat would be easily conducted through a brass case it would remain trapped inside a polymer one.

      • LG

        No, not really. In actual trials, the heat buildup in the weapon is greater because the case no longer acts as an electable, discarded, heat sink. Ceaseless ammo has the same problem. The heat buildup is accelerated, compounding problems of cook off etc.

        • nobody

          >No, not really. In actual trials, the heat buildup in the weapon is
          greater because the case no longer acts as an electable, discarded, heat

          …except the recent LSAT machine gun has demonstrated the exact opposite.

          • LG

            If the heat energy is not present in a pile of hot smoking brass past the ejection port, then it has to be within the weapon. the metal case merely acted as a heat sink and was ejected from the weapon. If the heat energy was not extracted by the metallic case, then where did it go?

          • I don’t think it’s comparable. The LSAT machine gun uses a swinging chamber which probably allows greater cooling even if heat transfer with each shot is greater.

          • LG

            Precisely, If the metal case is not utilized as a heat sink, heat dissipation has to be either dissipated in a different manner, engineering and/or material redesign. Either way, in a conventional weapon, polymer cases would produce increased heat deposition into the barrel and chamber. Again, look at the 243 Winchester vs. the 6 mm Remington. They are comparable ballistically, but the slightly longer case neck, i.e. heat sink, gives palpable longer barrel throat life. There is no free ride.

          • jay

            Lg, you need to go back to school. The heat in the gun, that you think the brass case removes, comes from the burning powder inside the case. Brass is a copper based alloy. Copper is very good conductor of heat. So. While when you extract the case, you remove some heat transfered from the burning powder, it was the highly conductive brass that allowed that heat transfer. The plastic LSAT case acts as isolator not conductor. That heat is not transfered to the weapon, but evacuated down the barrel. The reason that fresh ejected brass is so hot it’s because it’s conductive such a good conductor.

          • roguetechie

            From my reading on the subject it’s slightly more complicated than just polymer does or doesn’t remove the heat a metallic case does. For one polymer cases generally result in a drop in actual powder capacity versus an identical exterior dimensions metallic case. But, the burn efficiency in some polymer case designs is greater by enough that it can physically do as much or slightly more with less powder!
            Think of it this way, when you cook with pans that have steel handles you wrap the handle in a towel before you grasp it. But if that steel handle has a plastic AKA polymer handle you reach out and grab it without anything between flesh and handle. This is because metals are generally conductive whereas polymers tend to be insulators, so when someone says brass takes heat out of the system and polymers don’t… At best they’re not painting an accurate picture.
            personally I’m extremely interested in the throat erosion situation, and what kind of stress we’re going to now be putting on the 1-3 cartridge lengths beyond the chamber. Though, honestly I’m not worried about it all that much because of the various projects that are going to be ready well ahead of or just in time for brand new caliber(s) of CT small arms to use from day one of serious preproduction and large scale trials.

          • jay

            Go read what those who tested the lsat lmg had to say about it. You have an isolator between the burning powder and the chamber. So how on earth would the heat transfer be greater than in the case of a copper based case? Copper is one of the best conductors out there.

          • jay

            The plastic case of the lsat acts as isolator and doesn’t transfer heat from the chamber to the weapon. The brass is an excellent conductor, so it transfers a lot of heat straight to the weapon. Please forget the stuf you may think you know about the caseless ammo talked about in the eighties. Case telescoped Lsat is entirely diferit thing.
            The lmg tested by the grunts, some time ago, was head and shoulder better than the M249. Lighter, more reliable, more accurate, lighter recoil. Better in every way.

          • LG

            No. The heat has to be dissipated in some fashion. The metal case removes, transfers, heat from the weapon. If one moves the heat transfer farther from the chamber and into the throat if the barrel, then the erosion, vaporization, of the metal increases. Look at the increased throat temperature in early smokeless powders with the tremendously increased throat errosion. Go shoot 2k rounds from a 243 Winchester and the same number from a 6 mm Remington and then borescope the entire barrel. If loaded to approximately the same ballistics one usely slightly more of the same powders in the slightly larger 6 mm Remington case. However the throat errosion in the 6mm Remington will be much less than the 243. Do not take my word. Go to the benchrest shooters who are the true experts in barrel and action performance. Heat sinks work. Also as case diameter more closely approximates bore diameter, for a given case length, powder burning extends farther down the barrel with increased errosion and muzzle blast. Do not trust me. Go ask a truly hard core benchrest afficinado. Any engineering modification has its own inherent tradeoffs.

          • jay

            Most of the hear is coming from the burning powder inside the case. If the case is made out of a good conducting material, like the brass, you have a huge amount of heat being transferred from the burning powder, through the conductive brass case, to the chamber. In the case of a plastic case, a lot of that heat is not transfered to the chamber, but ejected out the barrel.
            To learn basic thermodynamics, you don’t have to go to the range, but to school.

            You are confusing the case telescoped LSAT, with the caseless ammo. With caseless ammo, you have the propellant in direct contact with the chamber. Very different scenario.

          • LG

            No. If the heat is not removed by the case then where must it go? The polymer case is not a dewar. If the heat goes into the barrel, then barrel errosion and heating increases thus necessitating more frequent barrel changes without using new barrel materials. This is with conventual design weapons. As Nathan pointed out and you may have missed, the LSAT does not have a conventional chamber.

          • jay

            You simply get less heat transfered to the rifle with a case made out of isolator instead of conductor. Basic termodynamics.

          • LG

            No. The heated gases from propellant combustion has poor heat tranfer properties. The barrel has to correspondingly heat up more. If you look at cases with the primer flash conducted to the front of the powder rather than the rear, such as the late great Elmer Keith did with the 50 BMG there is less combustion in the barrel and less muzzle barrel heating. If the case were a perfect heat sink then the propellent burn would not have the increased pressure of increased heat and the bullet would not leave the barrel. The more heat that is dumped into the barrel, the greater the errosion for conventional materials

          • jay

            You are clueless. From heat management point of view, the metal case has zero advantage over polymer case. Everything you posted up there has nothing to do with the actual case material, but with the cartridge architecture.

    • Phil Elliott

      I don’t think a difference of .018 between the .243 and the 6mm Remington is going to have a significant difference in heat dissipation. Bbl. life is probably affected more by the type of bullets and the twist rate than the miniscule amount of Brass difference.

  • I’m just gonna let someone else be the guinea-pig on this one……

    • wzrd1

      I believe that the Guinea Pig is the US Army Marksmanship Unit.

  • Kivaari

    Once again the military is looking at the 6.5mm bore as being ideal. Only a 125 years ago the firearms designer came up with that.

  • Kivaari

    I thick the shoulder and neck, should be 3 times what it is now. If its a new rifle the chamber could be made easily. Where the cases fail is the next fractures. Make ’em thicker.

  • Zachary marrs

    So in the end, whatever happened to that pcp stuff?

  • guest

    I am all for moving ahead in technology – no matter how “unorthodox” it may appear, as long as it justifies itself in the long run.

    But I have several issues with this design:
    1) All brass cases have some kind of crimp. Proper, consistent crimp is needed for precision as it is crucial for that first moment when the powder is ignited and the bullet has not yet started to move out of the casing. How is this done here? Glue?
    2) Brass has the unquestionable advantage of rigidity. Magazine lips, velocity of the bolt and the stresses on the case during cycling etc all are dependant on the case not giving in. What happens here?
    3) Polymers are fine and have good strength, light weight etc. But compared to most commonly used metals the critical temperature point at which they get structurally weaker and melt is much lower. How will this case perform in a high temperature environment? (several mag dumps, new mag loaded cartridge left in hot chamber)

    IMHO the best solution as far as weight goes are aluminum cases, from the same aluminum soda cans are made of. It is ridiculously light, will permit to be “fireformed” once and seal just as well as brass, and unlike polymers it will take a lot of heat. I know CCI and Barnaul make aluminum ammo, just sayin…

    • 1.) PCP used some kind of glue/sealant. I don’t know about MAC.

      2.) Polymers can be very rigid; I don’t think that’s a major issue so far.

      3.) Some polymers melt at relatively low temperatures, but some do not. For more information, I’d ask an actual materials scientist.

      Aluminum actually is problematic, as it is flammable and not as strong as brass. It is also a strategic material that you’d rather not be throwing away with every shot. The aluminum cased ammunition that exists is typically low pressure stuff where these risks are minimized. Not ruling aluminum out (neither is the Army, BTW), but it’s not trouble-free, either.

      • guest

        Well if it did have enough structural strength, it would not be torn in half would it?
        Of course there are high temperature polymers, especially Teflon which could probably take most heat of them all, but gluing metal to polymer is always a son of a gun, no pun intended, hence a big “?” about the whole deal.

        Sure alu it is strategic. Tell that to Coca Cola, Pepsi and the whole gang. It’s strategic until it’s not. As far as flammability is concerned it does not matter. It only burns when powderized enough, and does not burn in any way like Magnesium and the like. Being a great heat conductor the issue is a non-issue, as far as a the microseconds it takes to fire off a round.
        As far as strength goes again – aluminum cans are hard as f***. There is a huge difference in the alloy that is used on the rails of your garded door, of what the Apple Watch is made of and the soda cans. If it’s good enough for virtually all other gun parts, aircraft etc then the question is only what specific type of alloy to use.
        The only actual disadvantage with all aluminum alloys is the relatively weak plasticity which leads almost instantly to hardening and fatigue cracks and micro-cracks when it is molded, so from a manufacturing POV making a good aluminum case would involve not one but several heat treatments to have the cracks occur while fireforming (firing) instead of having it pre-stressed during production, thus keeping the case from splitting when the round is fired. Another issue is lubricity that aluminum almost does not have vs brass, but then alu cases like steel cases should (or rather must) be coated, as there will definately be an issue with galvanic corrosion – alu case and X metal type for bullet and primer, so if Teflon covered rather than laquered then Bob’s your uncle.
        But as I said if it has been proven to work by two companies each on different continent then its probably works, and is definately simpler than re-inventing the wheel and having a case made from two separate materials with vastly different properties.

      • Secundius

        @ Nathaniel F.
        There’s Aluminum Oxide (Pure), Bismuth, Cork, Fiber-Glass, Graphene, and Nickel…

  • Vitor Roma

    I think the 7.62×51 needs to be replaced more than the 5.56. There are many 6.5-7mm cartridges compatible with the .308 action that offers superior BC and energy retention with less recoil and a slightly lighter barrel and ammo. The 6.5 Creedmoor has an amazing performance.

  • Phil Elliott

    Any Ballistic results from the .264 USA posted anywhere?

    • In the original article on .264 USA, I link to a request for ammunition which specifies performance figures:

      SPECIFICATION: Develop, load, and deliver mature test sample ammunition of two defined intermediate caliber cartridges with lightweight (threshold), polymer (objective) cartridge cases representing a reduction in cartridge case weight over conventional brass cartridges cases of more than 20 percent (threshold), more than 30 percent (objective). Two cartridges will be provided: 264 USA and 277 USA. The contractor will furnish technical and performance information, delivery time, and unit cost for a quantity of 100 rounds (test lot for verification) and then 1,000 and 2,500 rounds each (production lots) of 264 USA cartridges loaded with the 107 grain .264” diameter Sierra HPBT MatchKing projectile (PN 1715), 264 USA cartridges loaded with 123 grain .264 in diameter Sierra Hollow Point Boat Tail (HPBT) MatchKing projectile (PN 1727) and 277 USA cartridges loaded with 135 grain .277 in diameter Sierra HPBT MatchKing projectile (PN 1833) assembled in a lightweight cartridge case. The cartridge case head stamp shall include the cartridge designation (264 USA or 277 USA) (threshold), production lot number (objective), and manufacturers marking/symbol (threshold). The completed cartridges shall adhere to the applicable accompanying dimensions and specifications. The overall length of the cartridge shall not exceed 2.650 in. These cartridges will function in modified, purpose-built AR-10 rifles and magazine in the semi-automatic and fully automatic modes of fire with and without the use of a muzzle-mounted signature suppressor. When fired from a 16.7 in test barrel, the 107 grain 264 USA projectile shall achieve a muzzle velocity of 2875 fps +/- 50 fps with an extreme muzzle velocity spread of no greater than 50 fps (threshold), 25fps (objective). When fired from a 16.7 in test barrel, the 123 grain 264 USA projectile shall achieve a muzzle velocity of 2657 fps +/- 50 fps with an extreme muzzle velocity spread of no greater than 50 fps (threshold), 25 fps (objective). When fired from a 16.7 in test barrel, the 135 grain 277 USA projectile shall achieve a muzzle velocity of 2527 fps +/- 50 fps with an extreme muzzle velocity spread of no greater than 50 fps (threshold); 25 fps (objective). All ammunition will provide 1.5 MOA (threshold), 1.0 MOA (objective) accuracy to 600 m (threshold); 800m (objective). The average chamber pressure shall not exceed 55,000 psi +/- 5,000 psi for ten rounds fired. The port pressure should not exceed 16,000 psi +/- 300 psi. All cartridges will be assembled using Federal 205MAR small rifle primers (threshold). The propellant used should induce a flash retardant (threshold) and provide minimal (threshold) to no (objective) muzzle flash from a 16.7 in barrel with no muzzle device fitted. All cartridges shall include waterproof neck and primer sealant (objective). All ammunition should meet generic SAAMI rifle ammunition test standards (threshold) and MIL-STD1461E, MIL-STD-1168, MIL-STD-709D, and MIL-STD-636 specifications (objective).

  • Secundius

    Heckler & Koch had the same problem with their 4.73×33 G-11 Assault Rifle design in 1968 through 1990. The actual bullet was a .185-caliber 4.7×32.83mm Nitrocellulose Cartridge, the bullet was actually seated inside the cartridge itself. The problem was that the Nitrocellulose Cartridge remained stable, up to ~ +356(deg)F. But once the barrel reached ~ +536(deg)F, the Rounds started Cooking Off inside the barrel. The Program was cancelled in September 1990…

  • Brent Blanchard

    What’s even more important is how far forward the brass portion extends into the chamber. That high-pressure metal gasket function is NOT to be taken lightly! IME, anything forward of the ammo-identification M852 case cannelure can fail without serious threat of gas leakage–that’s about a half-inch forward of the casehead/bolt face. A nice flexible metal strong enough to hold the gas must be able to bridge the transition from the portion pressed against the chamber wall, to the more rearward portion which does not expand. The PCP stuff appears to foolishly depend on the polymer to seal that area.

  • Unlucky Eddy

    Glad I never bought these kinds of rounds.. I had terrible luck getting steel case out of my revolver…