Just the way I went about it . . .
Found a flintlock Traditions Deerhunter for 100. Seems to go bang every time.
Replaced the plastic stock with wood for 10.
Then got a 31" long drop-in fifty smoothbore tapered octagonal to round barrel for it. Been too long so I'm not sure how much that cost. But once handling it I found the light weight and length were really very nice. I'm certain that a suitably tapered forty bore rifled barrel would handle the same.
Still have the original rifle barrel and it might become a longer tapered small bore.