Thanks 3DogMike for the comments on IMR 3031. When I first started with these rifles in the mid 70s it was the go to powder due to it's burn rate. It was also the worst powder to try to regulate a double with, half a grain +/- and three inch groups would go to 12 inches, tear your hair out at times. Fast-forward to Autumn 2005, Sherman Bell wrote an article in "The Double Gun Journal" entitled Finding Out For Myself - Express Rifle Pressures, Nitro for Black. In it he pressure tests four powders, including 3031. Beg, borrow or steal this one along with his summer 2011 article in the same magazine on case fillers, both are worth keeping in your reloading file.
Best