No, you're not limited to BP only. In the trapdoor design (even new repros) you are limited to the pressure of the cartridge. (Sorry , but not sure of exact numbers.) But the Marlin and Ruger load levels are too much.
I try to stick with the 405 gr. LRN at (or around) 1300 fps. This is close to the original BP loads, and within the limits on pressure.
Wyman
P.S.- I do know where there is an original rifle (you may be partial to a carbine, I don't know) for sale in central Georgia for $749.00. It's not mine of course, but it is at my favorite gun shop. Great place with a great reputation. Being an antique, it can be shipped straight to your house. No FFL required. PM me if interested. I had been thinking about getting it, but the housing market sucks right now, so no chance for a while. The shop is worth visiting if it's not too far (~ 1.5 hrs from N. Augusta) . Just plan on taking more than enough money, for all the other gotta haves. They have plenty of those if you like Mil-Surps.