.44 mag

[Nuclearcricket]

I would guess somewhere in the 30 to 35 gr volume of powder would work. I would use 3f. In my 44-40 I just use powder and bullet and dont have any problems with either a hand gun or rifle. Just adjust your powder volume so that when you seat the bullet it compresses the charge just a little and you should be in good shape.
One thing you do want to be mindful of is the bullet lube. You want to use lube made for black powder and not the hard lubes for use with high speed smokeless. SPG, DGL and some of the home brews all will work good. That rifle with BP should be a fun thing to get out and enjoy the great outdoors with.
Sam