That's not quite correct. It's very close in ball ammo, and the T-65 design intent was, indeed, to try to match the '06 ballistics closely in a more compact package, with the .308/7.62×51 brass resulting.
The last M2 loaded was a 150.5 grain ±1.5 grains projectile driven to 2740 fps at 78 feet (2508 ft-lbs). M80 is a 146.6 grain ±3 grain projectile loaded to 2750 fps at 78 feet (2461 ft-lb). I don't have BC's for the lighter bullet, but if it is close to that of the 150.5 grain bullet, working backward will find the M2 at 2800 fps at the muzzle and the M80 at about 2810 fps at the muzzle. (Both tested in 24" test barrels, AFAIK.) That 2% difference in energy tips the advantage slightly to the '06, but it won't be felt by the target. It takes about 10% difference for nerve endings to notice a difference in KE.
SAAMI rated the .30-06 at 50,000 CUP and the .308 at 52,000 CUP, originally. Today the percent difference has shrunk slightly (60,000 psi vs. 62,000 psi) using the more accurate Piezo transducers as standards. SAAMI's practice of rounding pressures to the nearest 500 units may be responsible for the difference not being proportional in the two systems. In modern rifles capable of handling the same pressure in both, you can get .30-06 up another 150 fps or so over .308. Especially with bullets 200 grains and up, the extra powder space in the '06 has the advantage. But the military had to limit powder burn rates in .30-06 for the Garand gas system. That's why you hear some powder move when you shake the M2 ammunition: empty space. The burn rate constriction prevented the Garand from taking full advantage of the cartridge's potential, which requires filling it with something slower than the military powders. Nothing to stop the handloader from doing that, though, unless he's loading for a Garand.
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Last edited by Unclenick; November 28, 2011 at 06:00 PM.