I wouldn't say it is an elongated version of the 43 mauser as it is a straight wall case. Hence using 400 nitro express brass.

Unfortunately, load development for it came to a screeching halt as I determined it was off the face. So until I spend a chunk of money to fix, it's on the back burner for now.

All put together it felt tight, but upon firing it showed the primers had backed out of the case head (As in excessive headspace, not high pressure) and when I pulled the forearm, held the gun upside down and shook it side to side it was then obvious.

So, in the mean time until I get it fixed, I'm learning to cast bullets and working on other black powder cartridge arms so when I get back to working on it, I'll be at the top of my game for dealing with odd black powder cartridges.