I would have no problem with shooting this rifle. Appears to be in wonderful condition.
How is the bore?

A bunch of 12.7x44R's ended up over here on this side of the pond and many were bought up by gunsmiths. They were re-barreled to .40 calibre, chambered for the .40/65 Sharps straight ctg. Many had the date of 1884 or maybe 1882 on the side of the action.

As this placed them firmly in the 'antique' classification, along with being chambered for ammo not available off the shelf. Thus, those people with prior arrests and convictions for assault in Canada & thus barred from owning firearms, could indeed buy and shoot these legally, as well as flintlocks, but not caplocks - go figure.

Such are the laws of the land - here.

Those .40/65 barrels were new GM sticks, I think, and shot very well indeed.