I’d be inclined to buy these and maybe cut them back to the length you like.
If the doubles are like the Pedersoli Gibbs and African Hunter, the barrel channel on the double stock is probably the same regardless of caliber, meaning, the barrel channel for both the .45 and .72 rifles are exactly the same.
Not rifled but a load of fun with patched balls I’d bet!
May drop right in.

https://auctions.thegunrunner.com/lots/view/1-4PUXRB/pedersoli-10-ga-percussion-shotgun-barrels