.303 Brit used nitro cellulose as a propellant. Mercury fulminate will make a primer. Both can be made in primitive conditions. No need for BP in firearms, but it does have it's uses....
For black powder cartridge rifles, the 44-40 lever would be the way to go. The cartridge was designed to seal the chamber when fired and much less fouling gets into the receiver.
Works well with the right black powder bullet.