Very good information regarding the internal cartridge differences and external chamber differences between military NATO spec and their civilian equivalents.
One other factor not mentioned so far in this thread, is that most military ammunition [ 7.62 NATO & 5.56 NATO ] is designed specifically to reliably activate gas operated firearms. This mean that, as well as chamber pressure at the instant of firing, residual pressure at the port a few microseconds later, is also a major consideration. The military selects the proper powder burning rates, and/or adds "retardant" to the powder, to ensure there is sufficient residual pressure at the gas port, for reliable operation of gas operated firearms.
Commercial .308 and .223 ammunition, especially loads with bullet weights grossly different from standard NATO loads, may or may not match these residual port pressures , EVEN THOUGH THE CHAMBER PRESSURES at the instant of firing ARE THE SAME OR CLOSE.
The larger caliber 7.62 /.308 Win firearms seem to have a bit wider range of acceptable powders/pressure curve than the smaller 5.56 / .223 firearms. In my testing, I found that the old AR 10 [ non adjustable gas regulator models ] and the M14 would function reliably with loads as light as the Remington "Accelerators", which used a .223 bullet in a .30 cal sabot to basically duplicate .22-250 ballistics. Amazingly, irregardless of conventional wisdom on twist rates and light bullets, some military spec rifles shot these ultralight loads into very tight groups. Unfortunately these very tight groups would usually be way off the zero for other loads.
I also had success in .308 bored rifles with the cheap BULK .311" Lapua 123 gr FMJ bullets, intended for reloading 7.62X39 Russian. These bullets were loaded into 7.62 NATO CASES to duplicate 7.62X39 ballistics [ at about 2450 FPS ] making for a very light recoiling load for "practical rifle" or three gun games. They were accurate enough for 200 yd matches, and would push a bowling pin 3' straight back.
As for heavy loads, due to the design of the gas system and rotating bolt, the AR 10 will basically handle any heavy bullet you can stuff into a .308 case that still fits in the magazine. The M14, due to the long relatively weak op rod, and the rest of the design parameters, is restricted to 168 /175 gr loads. If you want to feed your M14 a steady diet of HOT loads or HEAVY loads, there are ways to modify the M14 gas system to match these loads, BUT UNFORTUNATELY, THERE IS NO FREE LUNCH, and setting up an M14 to shoot heavy loads, probably means you have to give up some reliability with the lighter loads.
With the M14, it is not necessarily the chamber pressure that breaks things, but the excess residual port pressure, due to ULTRA slow powders or heavy bullets, that causes the problems.
The .308/7.62 platform offers amazing versatility, with commercial or military loads suitable for almost any chore, especially if used properly, within the original design specifications. The .308 Win/7.62 NATO platform can also be made even more versatile by hand loading light or hand loading heavy. However, if you want a gas operated firearm to work properly with your hand loads, you have to be aware of the difference between Chamber pressure and residual port pressure.
of course,
as with all advice you get for free on the internet,
it may be worth much less than you paid for it,
and,
YPMMV.
LAZ 1
