Half stock plains rifle. The wood is just too good to be that old. Hawken style patch box. The checkering looks out of place on a rifle of that style. IMHO it's a fairly modern build. The barrel looks mighty short to be 40 inches. This one is short for a Hawken at 33 inches.
It's got some "features" that are a bit odd for a plains rifle. It's
got a back action lock---a side lock would be more correct.
The trim is a bit too fancy and shiny, and as Hawg said the checkering
isn't typical. Good looking gun--but not quite typical mid 1800's.
And the more I look at that lock, the more I think it's a lock from
a Peabody, with a handmade percussion hammer on it. If so, that
dates it at after 1866, as the Peabody wasn't made before that.
I think it's a plains style rifle somebody put together with a Peabody
lock, possibly an old barrel, and a newer stock. Really hard to tell
WHEN---this type of work is still going on today. A GUESS???
Between 1935 and 1975. Probably closer to the later. You might post it
on some of the black powder forums and see if anybody recognizes it.