Pancho:
I cannot speak to your Trapdoor, but I have an 1874 Sharps Military Rifle in 45-70 that I just bought and it wieghs maybe just a hair more. I shot it off the bench a couple of weeks ago with Ten-X 405 gr. smokeless loads, and after 15-20 rounds, my shoulder was feeling it. BP loads in the same rifle with the same bullet are much better. I'm thinking of trying some Trail Boss loads to see if that makes a difference, and/or some 5744 as well. Congratulations on the Trapdoor.