DD - you may have something there - that hemispherical foot print is quite profound & could very easily be produced by a ramp on the end of the chamber. Winchester did exactly this with the .45 Colt M94 Trapper.
I had one of those and had to use the bulging case measurements when developing my loads, to keep them safe.

Old original balloon - head factory Dominion RNWMP issue .45 Colt ammo almost blew there themselves.
I'm down to a 1/2 dozen or 8 of those.LOL - should not have fired the 2 I did try for measurements only.