Originally Posted by Edd
I said I would be interested in seeing some data.
Here's some facts; you may consider it data. Or heresay. Doesn't matter to me.
I suggested to two people years ago they should lap the necks out of their .308 Win. full length sizing dies to a few thousandths less than a loaded round neck diameter so expander balls were not needed. They instead just removed the decapping pin, full length sized some cases then loaded them normally. Both noted bullets seated harder. When fired, case head brass had extruded back into ejector holes in the bolt face. Their normal reloads didn't. Pressure was noticably higher.
I and others who seated Lake City match ammo bullets a couple thousandths deeper to crack the asphultum seal to get more uniform release force noted they needed another minute or two up on the sights for zeroing at 600 yards. While the cracked rounds had less elevation spread from more uniform muzzle velocity and therefore were more accurate, it was low enough below normal ammo zeros to make a difference. Pressure ring diameters from new case to fired dimensions was greater by a few ten thousandths with regular ammo; rounds with their bullets reseated just enough to break the seal had less pressure.
Someone mentioned that few thousandths extra jump made up the difference. So we pulled some "seal cracked" round's bullets back out a bit then reseated them to standard length. They shot the same as those seated a few thousandths deeper.
While not laboratory-grade testing and measurement, the difference was noticable to folks holding the highest NRA classifications for competition.