Numrich shows the same replacement .50 barrel, #1372120, for all of:

Bighorn
Bk92
Little horn
Lk-93
Ll-II
USAK

https://www.gunpartscorp.com/Manufacturers/Muzzleloaders-43443.htm

Suggests that .50 barrels are interchangeable across these models.

Anyone have a decent .50 barrel from any of these that you want to sell? Send a PM.