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Posted: 7/2/2010 3:32:46 AM EDT
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Can someone point me in the direction of some technical information regarding the chamber differences between 7.62MM and .308. I have noticed that most of all the 7.62 ar's are actually chambered in .308 due to some chamber differences/concerns.
I understand the reasons manufacturers are doing this is because the .308 chamber can support both .308 and 7.62MM safely, but 7.62MM chambers are slightly longer and may cause a problem with higher pressure/thin walled .308. I have a new LMT MWS and it says it's chambered in .308 on the barrel, so I would like to find some technical information on the details of chamber differences. Also, I have a bunch of 7.62 NATO ammo that I would like run through my LMT and I want to be sure none of it will be a problem. Any help would be greatly appreciated. I looked for an FAQ regarding the differences in the ammo, but could not find one. Thanks! |
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If I recall correctly .308 is actually loaded to higher pressures than 7.62 NATO.
So if yours is chambered for .308 you should be GTG. I have owned FALs, HKs and semi auto M14, and have always used both .308 and 7.62 NATO interchangeably. However I do stick to the 150 to 168 grain rounds. My current FAL is most accurate (less than 2 MOA) with Winchester 150 gr Silver Tip. In your case I would recommend contacting LMT for guidence since they will know exactly how the chamber in your barrel is cut. Please be advised that there are certian lots of surplus 7.62 out there that are unsafe to shoot. Some Israeli and some from Chile I think. British, German, Portugese, South African and anything FN made is pretty good. Also the Aussie is very accurate but I doubt you will find any. Commercial .308 will be a lot more accurate than most of the surplus 7.62. Good luck and let us know what LMT says..... |
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I'm gonna go out on a limb and say while there is some diffrences in headspace between the 2, I'm not convinced the pressures vary much at all.
NATO specs specify a 147 grain bullet moving 2750 fps +/- 50fps measured 78 feet from the muzzle. Commercial .308 velocity is measured at the muzzle, so in order to compare accurately one has to add about 100 fps the the NATO spec to make both an accurate comparision. This means that the faster lots of 7.62 NATO could run close to 2900 fps at the muzzle and still be in specification. Now compare 7.62 NATO to .308 and ballistically they're identical. So how is this if .308 has 10,000 psi more pressure than 7.62 NATO? |
| Along with 7.62 NATO having a longer leade, the problem is with the case itself. 7.62 NATO uses a thicker case over commercial .308. Because the .308 case is thinner, it can hold more powder, which in turn allows for higher pressures. This is why most recommend not to use .308 in a rifle chambered in 7.62. There could be a stacking of tolerances that can cause issues (longer leade, thin brass, more powder, etc..) This is my understanding of why there are concerns. Surely someone will be along shortly to correct me if I have spoken incorrectly. |
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