What's your experience with black powder?
Most people, myself included, start with a black powder revolver. It's enjoyable to shoot, there's no recoil, and it has lots of history to it.
From there BP shooters move into rifles. Usually traditional caplock or flintlock rifles.
Other BP shooters focus on hunting - modern in-lines for Deer Hunting.
Another aspect of BP is loading BP cartridges and shotgun shells.
Regarding the Mossberg 500 BP barrel, I don't think you want all that weight on a gun if you plan to hunt with it. You might start with a traditional rifle or inline rifle. Get a feel for it and see if it's still worth pursuing that hard to find Mossberg barrel.
Personally I prefer traditional over in-lines for shooting. They bring me (in my mind) closer to the origins of shooting.