This is an original Army issue trapdoor carbine. I'm afraid the heavier load would be a disaster.bearcat6,
presuming that it's an ORIGINAL trapdoor 65-70 grain equivilent of PYRODEX, behind a 375-400 grain plain LEAD bulllet is HARD to beat as a DEADLY ACCURATE deer/black bear KILLER in the brush.
(fyi, for a much stronger modern action, my brother-of-the-heart loads a 450grain GC bullet behind a heavy load of smokeless @2200+FPS that would stop trannnasaurus rex. - he shoots it in a customized Mauser action for rimmed cartridges.)