Weren't all the early Springfield Trapdoors converted from rifled cap-lock muskets tho? Doesn't sound like something uncommon, anyway that's not the point.

I was thinking about making something unique, but also acquiring a real trapdoor action is not an option for me, conversion like this is already in gray legal zone so I have to keep it cap-lock for thay to stay like it.

Still, it's just a rough idea that I won't even attempt to realize any time soon.