A bb gun is not a firearm. A muzzleloader isn't considered one by the ATF. If its a thompson or nef with a rifle action, it is a firearm to the ATF even if it has a black powder barrel on it now, the receiver/action is the firearm and the barrel s an accessory. Conversely if it's a bp action with a rifle barrel on it, it's probably shrapnel 
State law cn be more restrictive though.
