Assume a SAMII correct chamber/good barrel. As said, neck dia close to chamber size. Fill the space (call this pre-bore) between mouth and bore to get/keep bullet centered in/on entering the bore. So. everything is/stays aligned. Base exits neck, expands some to fill pre-bore, then shrunk down to bore. Drive band(s) hit rifling. Tough alloy won't expand much in pre-bore. IMHO, at this point, PC can only help by having a flat base! Was some talk about trailing edge failure - NOT. Drag a putty knife across the putty, it leaves a 'dip' in the putty. Yes, gas can escape there. Lands don't 'cut' grooves, they displace metal to form the bullet grooves. And that metal tries to flow into the bore grooves! Tough alloy doesn't do that well so we over-size our bullets to force an excess of metal. Trying to fill that 'trailing' edge. At this point, IMHO, PC provides a higher thermal insulation coating that slows gas cutting. The rest of the ride, lower non-metallic contact friction (dust) reduces lead sticking. Tougher alloy is needed at HV to prevent basic shearing of alloy on the lands. Mr Gebson also notes the loss (and streaching of GC shank) using hard alloy. I suspect this is due to high temp (pressure temp) on the base with the added friction of the copper GC. I recovered the GC/base of a bullet - all that was left - the lead in the GC was re-melted (small ring) and rest was crumbly. GC was bent where it cut through 12ga wire. 308W @ 2400 fps, 165gr GC 4% Sb HT & PC.
I say tough, BHN actually measures the stess/strain curve at permanant displacement and is a SLOW measurement. The curve changes with impact fps.