Blain? - you can get replacement barrels from Track of the Wolf, or perhaps straight from Green Mountain, but they'd likely be cheaper from Track. I think the replacement barrels go to .62 smooth, or .58, .54 and .50 rifled - slow twist for round ball.
One of the fellows on another forum had Ed Rayle (I think it was) rifle one of the .62 cal. replacement barrels.
The replacement barrels already have the under rib, barel key lump and thimbles installed. For the work done, they are very cheap - under $200.00.

GM barrels are very good shooting barrels.