I'm not sure about your gun, but it wasn't that uncommon around the turn of the last century to reem the chamber on a single shot 44-40 to make a .410 2.5" shotgun. I know of 2 of these, one of which is to be passed down to me in the future. You mention rifling which makes me wonder if this is the case on this gun. Measure the chamber length prior to firing if that is your intention for this piece.