As a SHTF tool, I think you're really making a mistake. Sure, people once used black powder guns in the real world, but it was because they had no choice.
Let's look at the Cons:
-highly increased reload time
-dramatically reduced capacity
-dramatically reduced reliability
-vastly increased maintenance
-increased weight and decreased maneuverability compared to modern equivalents
Now, the Pros:
-incapacitate your enemies with laughter
I currently own only 2 BP firearms, a Dragoon and a rifle. Getting the Dragoon to fire all 6 cylinders without a malfunction is about a 2 in 3 bet. There is no such thing as "not cleaning it;" it must be disassembled IN FULL every single time you've pulled the trigger. BP residue gets inside to the small parts. You have to carry loose powder, or pellets, to have a reload. The stuff is hygroscopic as hell, and it's humid here.
Seriously, whatever space and money you were going to dedicate to a BP, dedicate it instead to additional supplies to keep your modern firearms in operation. More ammo, more spare parts, maybe even just another gun.
Now, if you just want to have a really, really good time, that's a different matter!!!