45-70 used to mean 70 grs of powder
and the 405 gr was the standard bullet wt, too. Was the bullet dead soft, or was it "hardened" a bit, by tin and antimony? that would increase bore friction, and maybe increase chamber pressure. I have no idea about black powder. Is the barrel wall thick enough to let it be bored out and relined? I'd back off the load and try that. Is the bore pitted? Clean it really well, and look at it carefully. Pits both increase fouling and hurt accuracy, they can raise pressures, too. It's not like it's a serious gun, after all. So why not just back off on the load. I think that there's some smokeless, or at least, non corrosive powders available today.
and the 405 gr was the standard bullet wt, too. Was the bullet dead soft, or was it "hardened" a bit, by tin and antimony? that would increase bore friction, and maybe increase chamber pressure. I have no idea about black powder. Is the barrel wall thick enough to let it be bored out and relined? I'd back off the load and try that. Is the bore pitted? Clean it really well, and look at it carefully. Pits both increase fouling and hurt accuracy, they can raise pressures, too. It's not like it's a serious gun, after all. So why not just back off on the load. I think that there's some smokeless, or at least, non corrosive powders available today.