I'm assuming you mean a pistol or revolver ?

If so I have 2, a Pietta Cattleman in.44 & a Westlake Armscor in .38 (Sorry but neither are for sale at the moment) both proofed for Nitro propellant.

Your variation request should be for a Muzzle Loading Revolver (or "Pistol" if going for a single shot, but that's another licencing can of worms) it matters not a jot to the variation what powder you will be feeding it.