HiBC,
IIRC, you mentioned in another thread getting that -0.006" head-to-shoulder number from the Precision Shooting Reloading Guide. I went back and read John Feamster's section in that book on gas gun reloading, and on Page 126 he says he tried -0.006" in his 308 Garand and found it didn't appreciably affect the case life he got so he uses it. Then on Page 129 he says he does not recommend setting dies up for -0.002 shoulder setback, but this not because -0.002" head clearance isn't adequate. Rather, it is because variation in case elasticity can cause the same die setup to resize cases with up to +0.003" variation in head to shoulder length resulting. Thus he figures that if you set the die up for -0.002", you may get a few that are actually +0.001" coming out of that setup. Those are what he is trying to avoid. So he set his die to -0.006 shoulder setback so that even his worst case wouldn't likely be closer than -0.003". It's -0.001" more than he strictly needs, but that was the safety margin he wanted. Avoiding alibi strings in matches is critical to match wins because when more than ten holes are in the target, the worst ten are the ones they count. But if a handloader checks every resized case and re-resizes the ones that are too long, he is good to go.