Looks like an 1840 to 1860 English rifle. The bolster seems 1/2 way between the rounded Hawken-type
and the flat with drip rail, English style.
On the other hand, there were rifles in the West, San Francisco made very similar, seems to me.
The full stock is odd, being full stock I think and likely in the earlier time estimate, therefore.
Can't quite explain why, just feels that way to me.