Nitro conversion cylinders are popular in countries where cartridge revolvers are severely restricted by the government.
These aren't cartridge conversions but cylinders designed to use loose smokeless powder in a cap and ball style pistol.
These all use shotgun primers instead of percussion caps. You can't just set smokeless on fire like you can black powder. You have to hit it with some pressure as well as ignite it. This is what the primer in a cartidge does.
http://www.westlakeengineering.com/4640/4694.html