I like the Berger method (or some variation of it based upon bullet profile). I also do coarse seating depth testing prior to powder tuning.
I have found that large variations in seating depth (=/>0.015') can cause enough velocity change to move into or out of a "powder node", which may be...
I'd go with the Winchesters. In my 6.5 CM, Federal and Winchester produced similar velocities while CCI lost about 30 fps. Upped my charge weight to get back into the node. The CCI primers just fall apart when I deprime. Maybe a bad batch, but they always go bang and my ES/SD is still good.
Try running the bore scope in from the other end of the barrel. You may find that what your looking at is mostly shadow. Carbon buildup often has a rough edge rather than a "clean, sharp edge" all the way around. Clean it well and look again.
How much interference fit on your neck to bullet, and what is your case fill? Sometimes a little tighter neck or a light crimp will help SD. I have also found that getting 95+% case fill helps a lot.
Shoot it at distance and see what happens. Vast difference between shooting at 100 yards vs 500...
I haven't played with them alot, but was using H4831. Won't win any races but it has been very accurate in a few of my rifles, in various calibers. Can usually get .5-.75 moa, even in factory rifles with a little work.
Elevation will affect your external ballistics and may get you closer to a more acceptable stability factor, but I doubt it will change your group size to a large extent. What distance are you shooting (assuming 100 yards).
I've shot several bullets that were marginally stable (1.26-1.31) and...
Nice shooting! Building a 20" 308 myself. Waiting on barrel, stock, muzzle device; but I have some of the SMK's and varget. Hope it does as well as yours! Thanks for sharing.
While not necessarily the most accurate firearms, alot of old muzzle loaders had 1:66-1:72 twist. Probably 1/4-1/3 rotation of the projectile and some were pretty darn accurate.
Not apples to apples, but illustrates the point.
As Ulimate Reloader,BAT, & Alpa demonstrated, custom chambers, actions, and brass can hide pressure well.
If your getting extraordinary speeds from comparable barrel length, your likely over max pressure no matter the signs