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.
I am quite certain that the argument you've already won in your head was entirely more satisfying to you than the one that is actually occurring.
Member of WR&PC,BASC & UBC