The original carbine loads were .45-55-405 if I recall correctly.
100% correct...I am glad you jumped on that one...far too many guys make that false assumption and give wrong advice. I have heard it a million times as well!!!!!!
As for a good load for the carbine...start low and work up.
-A good starting point is 50 grains of FFg. Then work up by 2.5 grain increments. Make sure you get a slight amount of powder compression when seating the bullet...excessive is not needed.
-Check the diameter of the bullets you are using as they come out of the mold. Some will cast at 0.458 to 0.463 depending on the age of the mold.
-Get yourself some Ox-Yoke Wonderwads in 45 caliber. USE THEM!!!!! They work. If you cannot get those, cut a 7MM Rem. Mag case just behind the shoulder being careful not to deform it. Sharpen the edge and use it as a wad punch with thick stiff felt...like and good cowboy hat. This device punches out a perfect 45 caliber wad. USE IT!!!! I cannot stress that enough. The trapdoor likes a larger bullet...MINIMUM 0.459". This is reality that was gained through thirty years of competing with the trapdoor in BP Cartridge matches
-Don't worry about magnum primers with black powder or trying to fabricate a "drop tube"...not needed in this case.
All of this information comes out of experience that was gained through over thirty years of competing with the trapdoor in BP Cartridge matches.
Just for your information, I used Lyman mold 457193, casting in wheel-weights which dropped a 420 grain bullet at 0.461". I hand lubed them and used a 54.5 gr GOEX FFg. Seated the bullets so the first driving band was visible, and gave it all a solid roll crimp.
My 1880 vintage Carbine would consistently group 1.25 to 1.5 inch at 50 yards...100 yards opened to about 3 inches. You will find that your carbine may shoot a bit high...this is normal as the sight was regulated differently.
Now get out there and shoot that critter and don't forget to post pics.
BTW...I have a serial number listing for trapdoor firearms and would look up your if you want. If so just PM the serial # to me and I will check.