We know for a fact that the case is still in the forward position when the neck and shoulder expands and adheres to the chamber walls. How do we know this? Simple --- chambers with excessive headspace have case head separation issues. Create a false shoulder or use the bullet to jam into the rifling and those issues go away in the same chamber.
Straight walled cases will sperate from excess headspace also. Straight wall cases have less of an issue mainly because most straight walled cases operate at lower pressures. A buddy had a Siamese Mauser in 45/70 that separated cases at higher pressures. No issues at Trapdoor pressures. I set the barrel back and that resolved the issue. The 458 Win. Mag and the 458 Lott are noted for having head separation issues.
I have a couple S&W 625 revolvers in 45 ACP revolvers that I shoot a lot without moon clips. One headspaces well without the clips but one definitely has headspace issues without a clip. At 45 ACP pressures it doesn't create case issues. Kick the pressure up to 65K (of course that can't be done in a 625) and I am willing to bet stretch and or separation will become an issue.
Most people believe that it is only the firing pin impact that pushed the case forward. That is only partly correct. The primer upon detonation pushes out and needs the case setback to reseat it. A primed case can be used as a crude headspace indicator. Not recommend for revolvers' since it may lockup the cylinder. Low pressure loads in bottle neck cases will set the should back due to primer pressure and due to the low pressure the shoulder does not get blown forward.
I have shot light 30/30 cast loads that didn't reset the primer. In that case the only bolt loading would have been from primer pressure.