When I shoot my 6PPC the case neck becomes larger than it was in the loaded round, even if the neck clearance is .0015. It snaps back after the bullet is released, but not all the way. It is this springiness that makes brass work so well for cartridge cases.
It may also be instructive to look at how cases get tight from one firing that is at a pressure that is excessive. The body of the case stretches out to meet the chamber, and then they both expand. In the case of a really hot load, this expansion moves them out so much that when the pressure is removed, the stronger, thicker steel snaps back all the way, but the degree of expansion has been so great that the brass has been stretched to a new dimension that is slightly larger than the chamber, and we sit there pounding our bolt open with the heel of our hand.
If the fit of the case in the chamber is too close, before firing, the level of pressure that produces a tight bolt lift, after firing, is reduced. a friend ran into this when he switched to Lapua brass for his 6BR. The original chamber was cut with a reamer that had been dimensioned for smaller Remington brass (He trimmed the Lapua for the shorter case length spec. of the older chamber design.) At first his gunsmith simply polished out the back of the chamber to a size that would smoothly accept new or sized cases, but when firing revealed that cases were getting tight at lower pressures than they had been with the Remington brass, the chamber was recut with a reamer that was designed for the Lapua cases, that had more clearance. Problem solved. That rifle is now being successfully loaded to some impressive velocities, with no tight bolt issues.
I used that information when, at the request of a friend, I specified the dimensions of a shorter than stock throated (allowing loading with bullets into the rifling, at magazine length) tight neck, 300 Weatherby reamer. Although the throat length, neck diameter, and shoulder location were changed, the body diameter dimensions were not. We started low in our pressure testing, and worked up gradually, loading at the range, with a single case, that we neck sized. measuring belt diameter as we went. After seven or eight firings, going past the point where there was an ejector mark on the case head (relatively large diameter ejector) to the point where the belt diameter moved the second time (firs time on first firing that showed no pressure signs), I could easily chamber the fired case. It was remarkable.
I am not suggesting that anyone repeat any of this, and take no responsibility if they do. The point of the story was to illustrate something about the nature of brass, and chamber design.
), shooting from field positions (for me it's shooting off shooting sticks) to become a better marksman. For me, this has yielded a much better return than trying to squeeze that last 1/8 to 1/4 inch reduction in group size spending hours on the bench which incidently I hate shooting off the bench. But as I said, I'm a avid varmint hunter not a benchrest shooter and I do respect those guys - shooting holes in those tiny little dots really is amazing. 
