The Indian "Post Mutiny" muskets were .65, I only know this because I have a weird obsession with them and tried to get Loyalist Arms to make a P59 .65 3-bander for me.
This was likely bored out to be a shotgun, for use as probably a Bird gun, etc. Either it was brought home after the war or bought as dirt cheap surplus from Bannermans.
It looks like theres some kind of rear sight? A P59 Musket would have a simple block sight.
It being a Smoothbore, if it were me , puts this in "parts gun" status unless it looks 100% shootable. Then maybe it's a fun shooter with round balls with some historical flavor .
You're right, Minie rifling is very shallow. Also maybe just the muzzle has rod wear.
Those barrels are made of basically Iron, make sure it's safe. I bought an original P53 years ago that someone bored to about .60ish caliber, it's not safe to shoot and I never got around to getting it lined.
I'd try to get it cheap if it were me and maybe see if you can drop a repro 3-bander barrel into the stock.