Heck, I suppose you could email Barry Corbin and ask him. There is a link
at the bottom of his website:
Can't tell a lot from the pictures. There is something a bit odd about
the area just above the lockplate. The drum/nipple looks way too long
for a normal muzzleloader. Markings? Caliber? Is the bore rifled---
rifling is one of those things that's hard to do, and totally worthless
on a prop gun.
ETA: (from the website)
LOG ROLE: Cardboard tubes of the sort used in heavy construction double as logs in Maurice's house. The fakery doesn't end there: The numerous rifles were salvaged from broken stocks and fitted with barrels constructed of wooden dowels or pipe.