Salmon,
I used to hunt with a White Mountain 54 Cal. with 425 grain Buffalo Bullets (preferred) or Maxiball 430 grain. I found I could use 90 grains of Pyrodex RS. Any more and the rifle lost accuracy. It was a great lightweight gun for sneaking around in the woods. It is a devastating rifle at short range. For your 50 Cal, I suggest shooting Hornady Great Plains 385 grain HB-HP bullets and as much powder as can be accurately shot.