[Bart B.]

Unclenick, in those picutes of groups, what's the "inch/Degree actual/theoretical" numbers for?

Indexing rounds in SAAMI/MIL-SPEC chambers does improve accuracy with .308 Win. ammo. Especially with those with more than .003" bullet runout. Runout less than that in such chambers shows little, if any, improvement when indexed compared to rounds perfectly straight. At least in my experience with M118 and M852 7.62 NATO match ammo.

Soft seating bullets is a boon to accuracy with handloads, as you mention. "Firm" seating is what I've used which is enough to hold bullets good enough for normal handling yet still seat them back a bit when chambered. These don't stay in the leade as the firm grip by the case neck pulls 'em out when unloaded rather nicely. I did this after my second need to push a bullet out with a cleaning rod down the upright barrel. But putting both back in tbvojhe case and shooting them resulted in their going to call on target.

The two .270 Win. rifles I've owned both had loaded round's head-to-shoulder length shortened a couple thousandths inch by firing pin impact. And verified by firing primed empty cases in them and measuring that distance. So, whatever bullet jump to rifling distance was set up by the bullet seater, it gets shortened a couple thousandths when fired.

Note the bullet's jump to rifling distance is best referenced to the case shoulder as it's hard against the chamber shoulder when fired. The case head is not touching the bolt face. There's a few thousandths inch spread across a bunch of cases' head to shoulder dimension. So, the "head clearance" dimension from bolt fact to case head when the round's fired will cause a small spread in the bullet's jump to the rifling even with the case head to bullet ogive reference diameter's length has zero spread.

All of which boils down to a couple of things that'll cause variables in the bullet's jump to the rifling; case headspace and shoulder setback spreads. It can easily be .005".