Originally Posted by Bullet bumper
Not totally true! It all depends on the dimentions of the sizing die , it may be tight at the shoulder diameter even though you are not bumping the shoulder angle much . This can cause some slap at the shoulder diameter , there is some shoulder clearance also. Belts do not alighn cases all that well cocentricly speaking. They mostly provide a headspace stop .
The process I outlined is not just about centering a case that is only one of the possible advantages.
A full length sizing die that's tight (small in diameter?) at the shoulder might make the case diameter at that point 10 thousandths smaller that the same point in the chamber. Sometimes new cases are that much smaller than the chamber is at that point but they shoot accurate enough to win matches and set records. I've shot 1/2 MOA 15-shot test groups at 1000 yards with both brand new belted cases as well as proper full length sized ones in a SAAMI spec'd chamber. A recent 1000 yard benchrest record was set with brand new .300 Weatherby Magnum cases.
Most folks getting best accuracy with belted cases don't set their shoulder back on a fired case more than a couple thousandths. This lets them "headspace" on the shoulder and that makes 'em center perfectly up front in the chamber regardless of how much smaller in diameter they are from the chamber. There's nothing to prevent them from centering very well and consistant.
No chambered cases align themselves dead center in the chamber's back end; there's forces acting on the case that prevents that. The extractor spring pushes the case towards the chamber wall such that it contacts the chamber opposite the claw when the bolt's closed on it. That's a very repeatable way to position the back end of the case. Even if the extractor and bolt face didn't touch the case anywhere, gravity would pull its back end down so it rested on the lowest part of the chamber. Note that a .001 inch off chamber center the back of the case is, that causes about .0005 inch offset of the bullet tip from dead center in the leade opposite of where the back of the case is; the round pivots on its shoulder against the chamber shoulder. But its very repeatable from round to round. I don't think there's any way the back end of a chamberd round could "float" dead center in the chamber.