Under FEDERAL rules, muzzle-loading guns don't qualify as "firearms" - even modern replicas. You can therefore own the kind of stuff Wild Bill Hickock carried, including front-stuffer double-barrel shotguns and six-shooter percussion revolvers.
Now, some states have rules more strict than the feds, some don't. So ownership of this class of old-school stuff is legal for you only in some states. I would seriously consider researching this, and whether or not this matters for street carry or home defense only.
If you are in this boat, I would score a replica of an 1858 Remington revolver in .44 caliber cap and ball. It has certain advantages over the equivalent Colt clones - topstrap is solid so the Remmie is tougher, and you can safely lower the hammer between two chambers so you can carry it six-up.
Failing that, your best home defense setup is probably a crossbow...repeating crossbows do exist