Kraig, your comment
The Army learned a long time ago, the most accurate velocity for the 308 in International Rifle Shooting was right at 2200 fps.
is familiar with me. What bullet weight was that used for; Sierra's 168's?
When around 1970 the USN team had problems with one lot of Sierra's 168's coming apart in the first 100 yards fired from MexiMatched M118 ammo in 1:12 twist Garand barrels, we called Sierra to ask about that. Martin Hull, their ballistician told us the US Army Rifle Team also had the same problem in their M14's 1:12 twist using the same ammo with that lot number of 168-gr. bullets. And the AMU's Barnett 1:10 twist barrels made 'em blow up closer to the firing line. So the Army gave them to the International Team folks for their 300 meter free rifles with reduced loads, those bullets shot very well indeed and probably the most accurate lot they've had. Sierra said they would replace that lot for both services; the Army kept theirs in the Int'l unit, the Navy got replacement ones.
I thought the M14's ammo was designed before the .308 Win. ammo was, not the other way around as your post implied. And I was told by LCAAP in the late '60's they used 21" and 21.5" barrels in their Mann accuracy devices as stated in:
The two Mann devices shown below bought from CMP sales have 21.5 inch barrels.
And while 2200 fps out of a 1:12 twist bullet will stabilize that 168 very well, heavier and longer ones have to be spun at a faster RPM. But lots of records were set and matches won with reduced loads in bolt guns at 200 and 300 yards with that bullet leaving in the 2200 to 2300 fps range from 1:12 or 1:11 twist .308 Win. barrels. 40 or 41 grains of IMR4064 under 168's was a popular reduced load.
Kimio, as long as the barrel whips and wiggles at the same frequency for every round fired, it doesn't matter; accuracy will prevail if the bullet leaves at the right place every time in the muzzle's whip arc. And that's typically just before the muzzle angle reaches the top and starts coming back down. Every free floated barrel whips and wiggles with the same exact fundamental and harmonic frequencys for its shape with every shot fired regardless of the load used. Stiffer ones at higher frequencies, whippier ones and lower frequencies.