I had some moderate loads of W820 jam the cases in a .45 Colt due to the VERY fine ball powder
being wedged between the brass and the chamber. The only explanation is that the pressure puffed
out the case neck before the front portion of the powder was burning and it flowed around the bullet
and then did a U-turn and flowed rearward between case and chamber. Hard to get the cases out,
powder not burned, but hammered really flat.
The relate to what Ian said.
Bill