A couple of points do come to mind. This gun may well have been made with oversize bores for use with all brass shells. In 1872 the 11/1 marking had not come into use. It could thus have started life of a size Just Shy of taking the 10 gauge plug of .775". The bore may not have been enlarged more than .005"-.006". This could well account for the thickness of the walls which would seem unlikely had the bore actually been enlarged some.030".
Technically though, yes it's out of proof. With that kind of walls, I wouldn't concern myself about that.
