Without a LOT more information, your question is impossible to answer. Find a local gunsmith that can give you some instruction.
No
who made the gun. how do you know it's a black powder gun ????
Follow all local, state and federal laws, place ad in newspaper, put on internet auction, pawn shop, gun shop, gun show.
Any of the PCP class of pellet guns or rifles. These are high pressure pellet guns that are capable of shooting a 50cal pellet.
about £678,900,46.58
bob the builder
barrett .50cal
It takes more than one shot, but if you have a Ma Deuce, hold down the triggers and sweep the gun from side to side, you can cut people in half with it.
It depends on local legislation
Two in the head with a barrat 50cal.
A couple of hundred meters.
Earlier gun than the Old West.