Since it is a .50 caliber use Goex 3f. This will work as the main charge and the pan flash. They sell special pan primer flasks. I would suggest getting one. The amount they put in is very small. I am not sure of the exact amount, just one press into the pan. Don't bother withsubstitute powders other then real black powders.

Also on my Lyman Trade Rifle I get the fastest ignition if I put the primer charge to the outside edge of the pan. It should not be right in front of the vent hole liner and never deep enough to reach the vent hole liner. You want a flash, not a fuse. Be sure and pick your vent liner before you fire. Some people use a feather, I use a vent pick.

Pay special attention to the flint or agate that comes with the rifle. Set it so the leading edge of the flint is about 1/16 - 1/8th inch from the frizzen. Put your rifle in half cock and then close the frizzen over the pan. Then you can see the gap you need.

The twist of the rifle will dictate the kind of projectile it will shoot best. Usually roundball work real good. Some people like .490 in their Lyman's others like the .495. You will have to try and see what your rifle likes best.

Good luck with your rifle.