24 November 2020
Trapdoors are notoriously oversize in the bore, they run .460 to even .463. This was noted to be designed to allow for black powder fouling.
Black powder: A soft bullet (30:1 Lead/Tin) with black powder that swells into the rifling is key. A black powder that does not leave much fouling in the bore after each shot; Swiss 1 1/2 Fg or Old Eynsford are a place to start.
You will need to develop a load for this distance by testing several bullets. I have molds from 300 to 525 grains and burned a lot of black powder testing the loadings. Although it takes several minutes more elevation, the SAECO 1881 mold, 500 grain round-nose government bullet, shoots better in my Sharps at 600 yards.
One consideration is using the RCBS 45-325-FNU mold at 200 and 300 yards with 55.0 grains of powder and extra wads to get less recoil over the course of fire. Best of luck, there is an undeniable charm shooting a Trapdoor.
Farm boy from Illinois, living in the magical Pacific Northwest