Beware of Commercial .308 and Fluted Chambers

My good friend CJ is a real whiz at building H&K roller locked stuff. Back in the day when G3 parts kits were affordable and flowed into the United States like water anyone could build one for a few hundred bucks. The problem however (aside from relative build difficulty) was that a true G3 spec chamber does not like commercial .308 Winchester ammunition. G3 rifles have an internally fluted barrel. This aides in extraction and prevents the possibility of an extractor breaking, and a shell sticking. The gas from the discharge is forced back and around the spent shell blowing it out of the breech. So, in the event an extractor breaks, or some ammunition failure, you will definitely get the spent case out of the weapon. With no extractor at all it will eject a round, or at worse cause a partial ejection which requires you to pull the cocking handle and drop the round out. The problem with this is it mangles brass. CJ found this out the hard way:


What you see here is what can happen. This was commercial .308 fired in a properly built G3; It is very dangerous and can kill you. You must use 7.62×51 NATO if you have a true G3 spec barrel. As you can see, the top of that round has the aforementioned flute marks – that’s what a spent shell looks like – it gets ruined. Now, if you notice… that’s only 1/3 of a shell, the rest of it was ripped off. Since commercial .308 is much thinner cased, when the round discharges the more pliable case expands and actually sticks in the flutes, and given the power of the round, the extractor literally rips the back of the case off, leaving the front of the shell stuck in the barrel. The next round was cycled in with such force, it actually slid itself through the remaining case neck. Luckily CJ noticed the rifle wasn’t in full battery and didn’t fire it (that would have been catastrophic).

H&K 91 rifles had a much looser chamber to allow the firing of .308 ammunition, and modern manufacturers of G3 pattern rifles seem to have adopted a similar chamber. Just beware of what can happen and be safe!

Special thanks to CJ for sharing this information and photograph.

Alex C.

Alex is a Senior Writer for The Firearm Blog and Director of TFBTV.


  • paul

    Do the CETME, which are a Spanish knockoff of the HK I believe, have this same problem?

    • jack

      You’ve got that reversed. Don’t ever forget it.

    • Murdoc

      Yes, and technically you could call the G3/Hk91 a CETME knockoff.

    • mrninjatoes

      CETME came before there HK. And jack is right…don’t ever forget that.

    • Yes all variations can have that problem.

      • JWR075

        When using the incorrect ammo, all firearms will have problems.

  • Jake

    Is it possible that that could happen with .308 win. in a PTR 91 GI?

    • It can if it doesn’t have the proper size rollers. The rollers come in several sizes to conform with specs. A good gunsmith can check that for you.

    • JWR075

      If your rifle is chambered for .308Win than it is ok to shoot both 7.62 and .308Win. I have the PTR 91 G.I. and it works perfectly with both, though the .308 seems a little more accurate.

  • PCP

    Correction, the G3 is a roller DELAYED-blowback not a roller LOCKED action like a MG42. The former is what causes problems, the latter shouldn’t be different on brass than any other conventional locking system like a rotary bolt or tilting breech-block.

    • It does lock. At some point the rollers must lock against the trunnion. Either term is widely used.

      • McThag

        PCP is correct. It’s roller delayed blowback, it’s never truly locked.

        • concur. Blowback action uses the force of the shell to move the bolt. The rollers just slow it down enough for safe extraction. Roller locking uses some other mechanism to retract the rollers as it cycles. in the case of the MG42/MG3 its recoil operated, where the barrel and bolt carrier move together till the rollers retract.

          • Anyone who wants to examine a true roller-locked action should pick up a Vz-52 pistol.

          • The G3 is a direct descendant of the MG42 action.

          • McThag

            No it is not. It really isn’t no matter what the internet says or how many people repeat it. The StG.45 was not in any way derived from the MG.42.

          • n0truscotsman

            Actually it is.

            The operating mechanism is a nephew of the MG42s action and is a adaption of it.

          • It’s not from the Internet. I’m very much into the history of WWII guns so I’ve had that information for years.

          • NikonMikon

            You’re wrong.

          • NikonMikon

            No, it isn’t. The MG42 has a roller LOCKED mechanism that uses a muzzle booster cone to move the whole barrel backwards which unlocks the rollers from the chamber.

          • M. Sage

            OMFG, it is NOT a direct descendant of the MG42 action. It’s a descendant of a Volkssturmgewehr development project that didn’t make it to production before Germany surrendered.

            The MG42 is a roller-locked short recoil firearm, the rollers are the only similarity in the two, and they operate in completely different ways.

            Tear a roller delayed firearm (HK91, HK93, MP5, CETME) apart and tear a MG42 or CZ52 pistol apart and compare the way the two systems work sometime. It’s completely different.

      • Nmate

        No, it doesn’t. The German term for it is “half locked”. The bolt is never fully locked into place. The rollers just delay the blowback action. The G3 isn’t a direct descended from the MG42 (emphasis on direct). It is directly descended from the StG. 45.

      • NikonMikon

        You are incorrect. There is no muzzle booster on a G3 to actuate the carrier. It is ONLY roller delayed.

  • Ripley

    I’d say from the pictures that the next round wasn’t far enough into the chamber to let the rollers lock in place and release the firing pin. It would just misfire. I’m also positive that the bolt assembly was designed with this scenario in mind.

    • Suburban

      I would not be surprised. The Kalashnikov rifles were designed not to fire out of battery – there is a tail on the bolt carrier that stops the hammer from hitting the firing pin. The AR-15 won’t fire unless the bolt is locked-up – the firing pin won’t protrude enough from the bolt face to hit the primer.

  • How on earth is there even enough room in the chamber for this to happen? How loose are they? Good grief!

  • Esh325

    The roller delayed blowback of the G3 really isn’t a suitable operating system for a rifle, especially a .308 rifle.

    • n0truscotsman

      In a nutshell yes. In fact, the cetme and its action werent even intended to be chambered in the overpowered 7.62 NATO cartridge

    • Nmate

      Which is pretty funny considering it is one of the more durable rifles that can be had. There are G3s out there with hundreds of thousands of rounds through them with little more than plus-sized rollers.

      Your statement could just as easily be applied to the FN FAL, another rifle designed with a less powerful cartridge in mind.

  • Renato H. M. de Oliveira


    That’s why milspec projos MUST have cannelures; tha rounds face enormous forces within the gun’s action – just imagine how much more powerful the HK 11/21’s actions are, to deal not only with the carts but with belts too? Or their barrels are not fluted?

  • Pepin the Short

    How about the CETME?

    • JWR075

      Any firearm that is marked as 7.62 should only use 7.62. However if your firearm says 308 then you can shoot both.

  • translation..”We’re HK, and you suck!!!!!”

  • The photo shows a case separation from the previous round that was fired and the bottom half ejected from the rifle and the top half remained in the chamber. The next round was fed into the chamber and caused the bolt not to go into battery. The cocking handle was pulled back to clear the malfunction and the live round pulled the remaining half of the separated case out of the chamber with it. It is not a problem with the rifle, its a problem with nickel brass. Nickel brass is not flexible like regular brass and can separate even after one firing. The HK91/G3 has been in service for over 50 years in some countries and the Germans over engineer everything. It’s not a problem with an HK made rifle. Home builds and cheap copies of this rifle may have issues. Cetme was started by a bunch of German engineers that escaped Germany after the war.

    • Jonny R

      On the money Tim 🙂

    • Suburban

      Hydrogen embrittlement from plating? It’s very rare that I find cracked brass shell casings, but I had a handful of twice-fired Federal nickel-plated brass crack on me. Steel casings are a different matter, they crack a lot more often.

  • Chris B

    the .308/7.62 ammo difference has been known for 50 years.
    I use a H&K 770 rifle with fluted CHAMBER for 30 years ( there is a huge difference between barrel and chamber boys ) without issues EVER.
    Cheap arse brass will do it.
    Buy quality brass and you will never have that problem, throw the shit crap nickel brass in the bin and leave it there.

    Most people can’t clean a fluted chamber – you need correct cleaning equipment that 99% of gunshops will not carry.

  • Why are commercial ammo cases so thin in the first place?

    • The nickel cases are brass with nickel coating, making the brass really thin so the overall thickness or case volume remains the same as a regular piece of brass.

    • JWR075

      The bigger issue is why would you shoot a higher pressure load out of a firearm whose chamber is designed for a lower pressure load?

  • Duray

    How exactly would this “kill you?” Seems a bit inflammatory. How would it go into battery when the first case neck obviously stopped the second one? A .308 neck is over a quarter inch long, so your HK would have to fire a quarter inch out of battery. Even then, I doubt it would kill anybody except the person being shot at.

  • Major Nav

    SAAMI spec for a 308 is 60000 psi vs 52000 for 7.62×51 and the shell sizes are slightly different.
    Never fire 308 in a gun that is stamped 7.62×51 only. However, 7.62×51 in a 308 rifle is fine.

    • Nmate

      That is not at all accurate. Those are 5.56 NATO/.223 Remington pressures, not 7.62 NATO/.308 Winchester pressures. The chamber pressures for 7.62 NATO and .308 Winchester are roughly 60,200 PSI and 62,000 PSI respectively. You can shoot .308 Winchester in a rifle marked 7.62x51mm NATO without worry.

      • Major Nav

        The casings have different dimensions. 7.62 is shorter. Again if it is not labeled for both, the manufacturer is trying to tell you something. Keltec RFB is a case in point. Labeled 7.62 only and carries a warning not to fire .308.

        • bob


          Go away Major Nav. You’re DRUNK with misinformation!

          • Major Nav

            Just one cotton-pickin minute there Bob, check your reading glasses…

            From the Kel-Tec website RFB manual:


            Ammunition Use
            The Kel-Tec RFB is designed and chambered for the 7.62×51 NATO Cartridge ONLY. The Caliber is listed on the left side of the rifle above the magazine well. Do not use any other ammunition.

            All warranties, expressed or implied, are voided if faulty or inappropriate ammunition is used. Kel-Tec CNC, Inc. will not be liable for property damage or personal injury in such an event.

          • matt

            KT has repeatedly stated you can shoot .308 from a RFB. I’ve put over a thousand rounds of .308 thru mine.

          • Major Nav

            Ah, the old hearsay versus what the manual actually states. Which will hold up in court. Site your source.
            I have heard KT President say just the opposite and it is the reason they have not sold many more of these awesome rifles. I would have bought one years ago.
            Like I said at the top of this, the barrel will withstand the additional pressure and there is a risk of case separation, but in a semi-automatic, the action may suffer some additional abuse.

  • Wouldn’t have fired out of battery. I have an HK91. Never had that problem with it and commercial ammo either, but hey, first time for everything.

    • Giolli Joker

      That’s what I was wondering… would it really fire out of battery?
      I was doubtful and you confirm my doubt…

  • Anonymous

    There’s a lot of fear-mongering on this blog entry, and damn little actual information from people who know what the hell they’re talking about.

    From statements like “fluting ruins the brass” and “G3 rifles have an internally fluted barrel”, I can pretty well surmise that certain individuals have no idea what the hell they’re talking about.

    Those of us who have shot more than three magazines through a rifle, have some clue what a case head separation is, what it indicates, and potential problems it can cause. Sorry, but this post isn’t even wrong.

    • PCP

      To add on that, the problem with the G3 and other delayed blowback guns isn’t the fluted chamber per se but the fact that the brass is being extracted while still under considerable pressure, that is what gnaws your brass. The fluted chamber is actually necessary in order to the case be extracted without being torn apart; they function by venting some gas around the brass in order to “lubricate” it so it can be safely extracted. Some guns like the Five-seveN avoid that by having their ammo coated by a thin layer of lubricating polymer, that why it’s a very very very bad idea to home-reload (and be tempted to hot-load) 5.7x28mm if you intend to use in a Five-seveN (I’m not sure about the PS90 but I wouldn’t push my luck).

      • strongarm

        FN FiveSeven also uses a movable barrel travelling at half speed of
        recoiling slide to obtain a problemless extraction and in fact, it might
        be provided for not to use a fluted chamber along with getting proper
        cycling of pistol with very light projectile.

  • the chamber is fluted. well, so is the barrel, but we call those flutes the lands and grooves

  • Raoul O’Shaugnessy

    “Internally fluted barrel”..huh? wha..I dont even…..How about ‘fluted CHAMBER’.

  • G3-A2 FS

    The H&K G3 has a fluted chamber, (not barrel) and a delayed blowback action (using rollers) There is no problem firing .308Win ammo in a 7,62 NATO chamber.

    • JWR075

      308 ammo should never be shot in a 7.62. the 308 operates at higher pressure limits compared to the 7.62. Much like the 5.56 and 223.

      • Tuulos

        As long as you don’t shoot some hot loads (like some hunting ammo with high pressure) you’re completely safe.

        • JWR075

          All commercial .308 is loaded to higher pressures than the standard 7.62mm. Now so of the reduced power loads might be differ but the majority of .308 do in fact operate in the higher pressure category.

          • Tuulos

            The maximum pressure for .308 is 430 MPa or 62,000 psi while they are 415 MPa or 60,200 psi for 7.62 x 51mm. Couple of quotes from Wikipedia:

            “According to the official C.I.P. (Commission Internationale Permanente pour l’Epreuve des Armes à Feu Portatives) guidelines the .308 Winchester case can handle up to 415 MPa (60,190 psi) piezo pressure. In C.I.P. regulated countries every rifle cartridge combo has to be proofed at 125% of this maximum C.I.P. pressure to certify for sale to consumers.”

            “Although very similar to the military 7.62x51mm NATO specifications, the .308 cartridge is not identical and there are special considerations that may apply when mixing these cartridges with 7.62x51mm NATO, and .308 Winchester chambered arms.[5] Their interchange is, however, considered safe by the Sporting Arms and Ammunition Manufacturers Institute (SAAMI).[6]”

            As the first one shows, the pressure the .308 case can handle is a little lower than the maximum pressure 7.62 x 51mm rifle.

            Second quote shows that even SAAMI consider it safe to shoot both out of both.

            Also since we don’t really get surplus 7.62 x 51mm in Finland I’ve only shot .308 out of my L1A1 completely trouble free.

      • Dan

        SAAMI says it is safe. Therefore it is.

  • firedigger

    I have a Heckler & Koch P7 PSP, which also has a fluted chamber. If I try to shoot aluminum-cased ammo like CCI Blazer, the cases expand into the flutes and will not eject. The gun does just fine with everything I put in it except the aluminum stuff.

  • Lance

    I agree I never liked the G-3 looked cool. But any weapon that’s that rough on its feeding and extraction has issues. Preferably a FAL or M-14 would be a better pick if you need a 7.62mm Battle rifle.

    • Nmate

      The G3 is superior to either. You could probably make an M14 more accurate with a lot of work, but a rack grade G3 is significantly more accurate and more reliable than an FAL or an M14.

      • I own an H&K91,FAL and Springfield M1-a1 At least with my guns the M1 was a little more accurate. They are all pretty equal.

      • Dan

        Would not say superior or significantly more accurate unless you care to eventually eat your words.

  • JWR075

    Sorry to tell you and your friend, but that is what happens when you use improper ammo. If the chamber is 7.62x51mm then it was never meant to use .308Win. however the .308Win can use the 7.62x51mm. The .308 operates at higher chamber pressure than the 7.62x51mm.

    • wait, now i tho9ught it was the other way around, i thought if your gun was chambered in .308 winchester, you COULD NOT shoot 7.62×51 NATO, but if you gun was chambered in 7.62 NATO you COULD shoot .308 winchester…..

      i know that you can shoot .223 remington out of an AR that is chambered for 5.56 NATO, but not shoot 5.56 NATO out of a rifle chambered in .223 rem, i figured it would be the same with the 7.62/308 round

      • Bear

        Oddly enough, and somewhat counterintuitive given Europe’s higher pressure standard w/ the 5.56 NATO (vs. .223), .308 is actually higher pressure than the 7.62 NATO. Odd, that is.

    • Dan

      SAAMI says they are 100% interchangeable. But what do they know, they only standardized the specs.

  • RP-in-TX

    My CETME is marked 7.62x51mm, but doesn’t say “ONLY”. The comments here have me confused. What exactly is and isn’t safe to use in it? I’m thinking I should stock up on 7.62×51 and give my dad all of my .308 for his Winchester 88.

    • Major Nav

      If it doesn’t say 308, don’t use 308. Some guns are marked for both.

    • JWR075

      Good idea. The 308 operates at a higher pressure limit and is slightly different in case design than the 7.62mm. Now you father can shoot 7.62 out of his 308 chamber without threat of damage, but you can not.

  • Alex C.

    Gentlemen (kudos if you are a lady in this male-dominated hobby and are currently reading this) I use the term “roller locked” and “roller delayed” almost interchangeably and for simplicity’s sake. I realize there is a semantic difference between the two operating methods, but when shooting or writing on the web I will say things like “the rollers are locked into the trunnion” or “throw in some +2 rollers so the bolt gap is fixed and the head will lock properly” so unless I am writing an extremely technical article I usually try and simplify things. That being said, PCP is correct in that the technical term for the Vorgrimler based designs is roller delayed. Anyways, the reason this was dangerous was that the rifle was nearly in full battery and had it been fired out of battery (and gone off) it could have caused a catastrophic failure, which when we are talking about firearms can result in serious injury. Obviously being fatally wounded is an absolute worst case scenario! All in all if you have a commercial G3 variant like an H&K 91 or a PTR 91 GI then you have a chamber that can handle both 7.62 and .308 just fine!

  • briggs83

    very odd. We shoot commercial 308 all the time out of British made and original HK G3’s without any issues such as the one discribed above.

  • RickH

    The brass is reloadable after firing. The striations will come off with cleaning, the biggest problem with HK’s is not using a case buffer to prevent denting during ejection. We reloaded these all the time back in the ’80’s.

  • Egregious Charles

    This is NOT a .308/7.62 chambering problem- it’s a brass thickness problem. Commercial brass is much thinner than military brass as a rule. CETME/G3 family rifles use a fluted chamber to allow some of the propellant gas to leak around the case during firing, before extraction, and equalize the pressure inside and outside the case. This is because extraction starts much earlier in a roller-delayed design than a normal gas-operated design, while the pressure is much higher. The high pressure would press the brass against the inside of the chamber (especially up near the throat and create huge friction. Meanwhile the base/rear end of the case has started the bolt backwards with lots of inertia. If the front of the case is still pressed against the chamber by gas pressure, the friction will anchor the front while the bolt and case base head backwards, ripping the case apart. With thin brass, the pressure can squish the brass into the flutes and seal them up, preventing the pressure from equalizing as it’s meant to. (Can also happen with flutes that have been sealed up by enamel from enamel-coated steel cases; to be on the safe side I don’t use any ammo coated with enamel or polymer or anything softer than brass).

    While at a carbine class, as an experiment, I fired over 500 rounds of commercial .308 from one of my PTR-91s without any cleaning or problems (though the action became EXTREMELY difficult to move manually). The way I check whether a new load is suitable for that particular rifle is fire one round from an otherwise empty magazine, then find the case and run my fingernail over the neck sideways to make sure it’s still nice and smooth and round. If the brass has been pressed into little ridges by being forced into the flutes, that is if there are ridges corresponding to the black stripes in the photo, then that load is unsuitable and a broken case with neck left in the chamber is likely if I continue to use that ammo. (If it fails with one, say my CETME, I get rid of it, I don’t try it with the others.)

    Someday I may have the bad luck to break a case on the first try, but I keep a couple extractors handy.

    In my experience, the flutes can’t get blocked by normal firing residue- they get flame-cut open with every firing. To block them you need to do it all at once with something like enamel or soft thin commercial brass.

    • JWR075

      No shooting ammo in a gun and chamber that was meant for a lower pressure round is a big problem, thus a 308 should never be shot in the 7.62 chamber. However if the chamber is a 308 then one can shoot the 7.62, kind of reverse from the 5.56 /223.

  • Garek

    I have a PTR-91 that has fired several different types of .308 which has had quite a few primers blow out the back and cartridges stuck in the receiver to the point of a stuck bolt and non-functional rifle. I have found this to be very frustrating. I “believe” Hornady works well (not happy about that due to price) and actual Winchester brand .308 doesn’t. I have noticed that ALL of my brass is pretty mangled afterwards. I LOVE my rifle and it’s DAMN accurate but totally unreliable without the proper ammo.

    I love some input wrt my issue (similar experiences) and any mitigation folks can think of.

    I want to load my own .308 that will work in my: .308 Commercial Barrel Garand, PTR_91, and .308 Savage Axis bolt gun. Suggestions here would be great as well.

    Thanks in advance.

    • Bucking the Futz

      odd….I have a PTR as well and haven’t had a single issue with it whatsoever

  • Major Nav

    It is a chambering problem. The 7.62 chamber is 1.645 vs 1.632 for the 308. When fired, the 308, which has thinner brass, can only expand .008. It needs to stretch .013 to fill the chamber. That is why it separates, fluted or not. Source

    • matt

      Headspace is not 1.632 for .308, it 1.640. Check SAAMI’s website if you dont belive me.

      • Major Nav

        Are all manufactured rounds, reloads, and rifles made to SAAMI standards?
        BTW, SAAMI standards for 7.62×51 is different the 7.62 NATO Standard. NATO From personal experience I can tell you that all “NATO Standard” rifles and and ammunition certainly are anything but standard.
        That is what the article sited above was referring to. Despite the SAAMI standard, those numbers are what he actually measured in the chamber.
        Regardless of the different pressures, as most rifle barrels are tested far beyond ammunition load limitations, the 7.62 is GENERALLY longer than the 308 but will fit, albeit snuggly, in an AVERAGE 308 chamber. An AVERAGE 308 will easily fit in the AVERAGE 7.62 rifle chamber because there SHOULD be extra headspace. The weaker 308 casing will expand to fit the chamber and in some cases, especially with alloys that don’t stretch as much as brass, the case will separate. (See photo above)

  • ian

    Last weekend, went out and shot old commercial/winchester .308 in a early 2000’s CETME. About rd. 15, bottom blew out of the 5 rd. magazine. Buddy’s forearm was blackened and reddened from it blew out the bottom of the mag, brass was mangled, i.e. couldn’t tell if it was one or two shells it was mangled so badly. I assume an out of batter explosion from commercial .308 in a fluted chamber. Previously had about 1220 rd of NATO through it with only without any such problem with this or other magazines. I’m taking my sample size of one, and I’m not going to shoot .308 commercial out of this, not worth it.

  • M. Sage

    Two points of fact here:

    One, as PCP notes, the G3 is roller-delayed. If it was truly locked, blowback wouldn’t open the bolt.

    Two, the fluted chamber is also there to delay blowback by giving a little “grip” to the brass as it expands into the flutes, which is probably why that thinner case got ripped in twain. The flutes aren’t to aid in extraction or prevent stuck cases or whatever you were getting at. That should be obvious by the partial case that was stuck in the chamber…