Original 44-40 caliber had a nominal bore of .427, but modern 44-40's use .429 so as to standardize the barrel with .44 Russian, .44 Colt (the modern version), .44 Spcl, and .44 magnum. Often folks will fit a 44 Spcl cylinder to a 44-40 SAA or vice-versa. If your uncle's gun has a .44 Spcl cylinder he can shoot 44 Spcl, .44 Colt (the modern version), or 44 Russian in it. You cannot fire 44-40 in it as the round will not fit in the cylinder.