The black powder rifle is a percussion cap muzzleloader that makes use of .50 caliber balls as ammunition. The black powder rifle has a unique reloading animation that involves the player character using a ramrod to push a lead ball down the rifle's barrel.
The black powder rifle may be obtained at the Philippi Battlefield Cemetery inside the museum in a display case along with other weapons and outfits.
One can be found at the civil war fort, Prickett's Fort, in a display case in one of the cabins near the parking lot
There is one at the Garrahan Estate. Take the elevator to Level 3, turn right, enter the door to the right of the stairs. locked display case (level 1 lock).
There is one at the Harper's Ferry on the east side in a small building. On the left as one travels from the fast travel location to the vendor.