Back when the 30 caliber belted magnums were "in" for long range matches, folks winning the matches and setting the recored did so with one of the following case types:
* Brand spankin' new cases, unprepped in any way except for running the neck through a die to uniform the mouth. Bullets were seated a tad longer than throat contact.
* Full length sized fired cases whose shoulder was set back a few thousandths, but more important, a second body die was used to size that step right in front of the belt back down to new case diameters. Otherwise, it interfered with consistand and uniform positioning of the back end of the case in the chamber at that point. Again, bullets were seated a tad longer than throat contact.
Here's the best example I know of comparing new cases vs fired ones. The picture below's a plot of 30 shots fired about 20 to 25 seconds apart. A .300 Win. Mag. case is full length sized in a standard .30-.338 die making its neck longer than a normal .338 Win. Mag. case is. Black dots represent Sierra 190 HPMK's loaded in 3x fired cases, neck turned and full length resized in a standard die whose neck was lapped out to 2 thousandths smaller than a loaded round's neck diameter. Then sized again with only a body die reducing the body diameter immediately in front of the belt back to virtual new case diameter. Red dots are Sierra 200 HPMK's loaded in new, unfired cases prepped only be resizing the neck for uniform mouth diameter. First a 190 was fired, then a 200, then a 190, then a 200...... until all 30 shots werre fired. I thought this would be a good test of how each load would perform in a 30 shot string of each.
Note the X-ring on this 1000 yard target is 10 inches in diameter. Compare these groups to what 1000 yard benchrest guns do for the aggregate of three 5-shot groups.