As a former competitive black powder shooter in the N-SSA (over 15 years worth!) I'd love to see some weapons of this type. That said there's a few caveats you might consider:
1) Realistic reload times of 20-30 seconds per round!
2) Pistols should use the realistic reload of 5 minutes for a 6 round revolver, however. (not exaggerated).
3) Damage should be absolutely grievous. Our modern weapons are great, but there's something magical about shooting a 58 caliber, 460 grain full wadcutter slug at 50 yards and just absolutely decimating something.
4) Don't forget percussion caps with black powder too
those made life much more liveable.
5) Let's get lee to get some good volumetric fog in so we can really 'live the experience' of shooting black powder and getting a massive plume of grey smoke after each shot
6) Add smell-o-vision to allow that wonderful aroma of black powder after a volley.
7) Add black powder as a seasoning for foods in game. In case you weren't aware, black powder is edible (It's primarily charcoal, potassium nitrate, and sulfur) and actually tastes pretty good on french fries...
All kidding aside, that gun looks real good. Probably one of your best models yet.