what causes this in chamber?

[bluebayou]

Yesterday, the guy to my left at the range was hitting me with these from a CETME:



Is the chamber oversized for reliability?  Why the longitudinal action?  I hope that the picture works.

[DWTim]

There are grooves cut in the chamber presumably to leave a place for the crud to collect.

EDIT: I believe the term for it is "fluted chamber".

[mjbgalt]

yep. fluted chamber. that's why .308 CETME is not compatible with .308 Winchester.

-MJB

[gunnut69]

All the HK type rifles have these flutes.. The actions are delayed blow back and unlock with considerable  pressure remaining in the barrel. To allow the case to release from the chamber walls easier and avoid ripping off the caseheads, they flute the chambers. It allows the pressure from the expanding gas to equalize on both sides of the case wall. This limits the amount of case sealing the pressure to the very rear of the casing.. It works very well as these, in the form of the HK91/93 are considered quite good as military weapons go.. The ejection does tend to be a bit shall we say, energetic!!  It makes the cases pretty distinctiv but doesn't damage them. I've even reloaded a few..  The 7.62 Nato and CETME rounds have about the same dimensions as the 308 Win, but if memory serves all were loaded to different pressures.. Now for goodness sake don't take anything from my memory as gospel...

[bluebayou]

I know that the CETME round is different from the NATO and the Winchester.  The CETME specific round is pretty weak it is more like a 7.62x39 in size.  This rifle that the guy had next to me was a 7.62 NATO rifle.  Dimensionally, it isn't close.  I was horrified to see the longitudinal lines on the brass and couldn't figure out what caused it.  Since reading your reply, Gunnut, I looked at an exploded view, I understand what you are saying.  Thanks.  I had always teased with the idea of getting a CETME, but after handling this guys I think that I will pass.

[gunnut69]

I went back a researched it a bit and your are partially right. The original CETME round was a shortened reduced power round. The second variant used 7.62x51 dimensions but at a reduced power level, although the bolt and return spring could be replaced and full power NATO spec ammo could function. The final version was made with fluted chamber and was made specific for the NATO spec 7.62x51 ammo at full power. This is close enough to 308 win dimensionally that the cases will interchange. Power wise the NATO round is a shade lower pressured but I've fored a lot of 308 in M1A and in the cetme/HK rifles and they work just fine. So!! If the rifle was spitting out empties as in the photoes it was likely a Model 'C' CETME or later and use of nato/308 ammo would be fine..

[bluebayou]

I looked up the history of CETME in an old Shotgun News and an encyclopedia that I have.  The original CETME specific round is goofy looking with brass the length of the 7.62x39 but a abnormally long bullet that makes the round as long as the 7.62/.308.  I also so the reference to an "underpowered" NATO size round.  Pretty interesting actually. 

I never would have guessed about the flutes in the chamber.  I still don't understand the point of the flutes if the bolt and spring are exchanged, but I am not a Mauser/CETME engineer.  I wonder if the G-3 has fluted chamber or if the Germans redesigned their kameraden design. 

[gunnut69]

The version (B Model I think) used the interchangeable bolt/spring assembly to convert to the Nato full power round. It didn't have the flutes! The flutes were added to the 'C' version or the 3rd version. The function of the flutes is to equalize the pressure on the brass case by allowing the pressure from the firing cartridge on the outside of the case. This allows less of the case to grip the walls of the chamber when it is fired and makes extraction a lot easier. The problem with these delayed blowback rifles is that extraction is very very quick and the rifle opens with some pressure still in the bore. The casehead is often ripped of and the body remains stuck in the chamber, requiring a stuck case removal tool. Fluting greatly reduces this possibility.. I'm not really familiar with the G3 but since it too operates with high pressure ammo I would speculate that it also has the fluted chamber. The HK's(rifle calibers) all do.. Another interesting tidbit,, The long bullet you spoke of in the original CETME round(7.62x40) was made of aluminum. It evidently worked OK but eventually they conformed to the NATO standard.. just as we have by switching to the M9 Beretta 9mm pistol. Of course if you follow the news the guys in the field (Afganistan and Iraq) are finding the 9mm an unreliable stopper and the Army is releasing specs for a new pistol(single or double action, single or double stack magazine, but in 45 caliber). One would have thought this lesson learned in the 1890's during the Phillipene revolt.

[bluebayou]

My armchair opinion is that since we always fight "this time" with the lessons of the last time that:

1.  Pistols were considered gentlemanly again, selective people carried them and they were of limited use
2.  Who used a pistol in the SE Asian jungle?
3.  Urban warfare is different and CQB makes a pistol a primary weapon sometimes
4.  Whoever's stopping power theory you subscribe to, the 9mm is small and weak if you compare apples to apples (FMJ to FMJ)

So......well, there you are.  I think that the old idea of NATO interchangeability is a bit outdated.  Not many countries stepping up lately. 

Personally, I love the fact that all of the mothballed M-14 type rifles have been shipped to Afghanistan and Iraq.  Urban warfare or not I would feel better having a 7.62 rifle around. 

[gunnut69]

I kinda agree but a very old rangemaster at the first qualification range session once told me that 'if you're gonna carry a gun, make sure it's big enough'. I carried a 9 for many years and saw a few failures.. the 45 kinda reduced the number of failures. A pistol is a kinda last ditch thing..so it's likely that a failure could be fatal!! I like 1911's and hydrashocks.. As to the 7.62 Nato, it seems that even in the urban environment of Iraq the .224 is a failure. The developement of the 6.7 or whatever by remington would at least seem to suggest.. Whatever you're carrying at least learn to use it well!!