I agree with BrentD that it is a Field's 1877 patent action with a Winchester barrel.
Rebarreling English falling blocks to American cartridges was pretty common practice once although this one looks to have kept its original forend which is nice.
I bet the heavy 30/40 barrel profile was made to closely match the rifle's original barrel profile or just happened to be close.
I would think that the original chambering would have been a 450 or 500 of one sort or another.
The Field action is exceptionally simple but of high quality and many of the high tier gunmakers of the day built rifles on this action.
Had I seen it, I would have considered buying it and rebarreling it to a more original chambering.
I have one in 500 No.2 and would not mind another at all but buying this one and then the rebarrel and proper sights would probably put the project at too high a cost. Does have nice wood though...
