I called Carl (who handled it in 1975 and 1976) and he couldn't remember, saying it was .52 or .54, bigger than .50. I called Carney Pace who also worked there, and he said he had an agument with Greg Roberts the Production Manager (or whatever his title was) about the bore size. They had slugged the bore and the slug miked .540, which was really the bottom of the grooves, which are .010 deep. Carney said it was .520 bore. I have Greg Robert's tracing and dimensions of the rifle, and the barrel mikes 1.175 ahead of the snail and 1.125 at the muzzle. I checked this at the Helena Museum last September and that is also what I measured. The barrel measures 33 1/8" long ahead of the snail. It is not tapered, .050" is the thickness of a dime, half that per side, Carl said you could make that much difference with a file.