[Bart B.]

Yes, I know how cartridges fit chambers, before, during and after firing. Furthermore, this is not about reloading. It applies equally to new, unfired cartridges, too.

Easy to learn after one knows how external and internal forces act on them. And what the forces are to start with.

I've tried to explain this. Have you tried what I suggested with the bolt and a primed case?

You can push a case into a stripped bullet seating die then watch its mouth center on the bullet chamber. Functionality the same as the barrel chamber.