From one of the competition shooters:
"If there is published 38-55 and 45-70 load data for Blackhorn I'm sure it would be safe to use in your H&R and trapdoors AS LONG AS BLACK POWDER PRESSURES ARE NOT EXCEEDED......ESPECIALLY IN THE TRAPDOOR.
Many loading manuals list data for low pressure, black powder equivalent smokeless loads for 38-55 and 45-70. I have gotten good accuracy using Unique and SR 4759 powders in both calibers. The 38-55 was usually loaded with bullets of around 250 grains, the trapdoor rifle was loaded with 500 gr. lead bullets, the carbine used 405 gr. lead bullets to help reduce recoil."