Originally Posted by Mada
I see what you're saying now. From the belt theres a little slope, Kinda like that took the case and crimped it inward to fit the belt onto the case... Thats were my case DOES grow fatter and therefore the die wont fit.
Is this because the case as formed itself to the size of my chamber?
Yes, that's what's happening.
When the firing pin strikes the primer, the case gets driven hard forward and stops someplace....
If a new case is fired, it stops when the front of the belt contacts the headspace ridge in the chamber, then as pressure builds and the case expands, the case head gets pushed back against the bolt face leaving a small gap between the front of the belt and where it headspaces in the chamber. Body brass expands in this tiny area making the step.
If a previously fired case is used, it may stop when the shoulder impacts the chamber shoulder if the shoulder's not set back when sized. So there's already a space between the front of the belt and the chamber headspace ridge so even more body expansion can happen. If the fired case shoulder's set back too far, that step/ridge will interfere with the chamber at that point and the back of the case won't be at the same place for each shot.
That darned ridge interfering with the chamber at different places around the case causes the barrel to whip differently while the bullet's going down the bore. The end result is the muzzle axis ain't always pointing at the same place relative to the aiming point on the target for each shot.
All this is why top competitive shooters using belted cases have always got best accuracy with new cases or cases double sized to get rid of that step infront of the belt. A recent 1000 yard benchrest record was set using brand new, unfired .300 Wby. Mag. cases. My own 1000-yard magnum when last tested, shot two, 15-shot groups at 1000 of about 6 inches; one group with new cases and the other with double-sized ones. I've never put 15 or more rounds in under 15 inches at 1000 using neck only, partial full length sizing or just a conventional full length sizing die by itself.