Browninghunter, what barrel?
All my .308 Win. match rifle barrels had standard minimum SAAMI spec chamber dimensions. No freebore at all past SAAMI specs, a couple with less for 155-gr bullets. A Sierra 168 HPMK seated out to a 2.84" hinch length would be a few thousandths short of rifling contact.
Dixie 'smith mentions heavy/high-pressure loads changing the ballistics enough that the bullet starts to do some squirrelly things, and accuracy falls over it. I totally agree. There's two reasons in my opinion.
All bullets are not perfectly balanced. When they spin at 140,000 to well over 200,000 rpm leaving the barrel, it doesn't take much out of balance to have the centrifugal forces upon muzzle exit make the bullet's flight path divert from the bore axis at the muzzle. This is the reason why all bullets fired at the same muzzle velocity have a very tiny spread in BC. Those most unbalanced have the most drag and that slows them down a bit more that the "perfectly balanced" ones. Most of the time, BC spreads are typically less than 1% in the best match grade bullets, but that's enough to be one of the reasons group size in MOA gets larger as range increases.
The other reason; supermaximum loads do not have their pressure curves as repeatable as normal max loads do. Nor do slower powders producing maximum velocity possible. That causes a greater spread in barrel time for the bullets; case mouth to muzzle exit. While the barrel whips at the same frequency for every shot, a spread in barrel time causes bullets to leave at a greater angular spread of the muzzle axis and the amount also gets bigger. While a little bit of spread in the right place helps accuracy, too much spread degrades it.
