Good luck on the search. Its not clear if this rifle is now shooting? Anyway, if an inertial puller was ruled out, Id go with a collet puller, commercial or home made hole in a halved block of scrap on a press without a die in it.

The nose would get a little distorted, but that could be measured and verified before you pulled the cartridge apart. If(?) you can get all the info you need, Id stick the powder and bullet back in and shoot it over a chronograph just for curiosity?

Even in an old time bore, I would suspect that for black powder pressure loads, soft lead bullets would bump up back at the throat, regardless of how an unfired bullet fits the muzzle? Anyway, continued fun with the journey.