The bore was measured by driving soft lead slugs through the bore,then you measure the slugs with a micrometer.Before you ask-yes,I repeated the measurements-4 times per barrel,only about 2 ten-thousandths(.0002")difference total.If this kind of dimension is surprising measure Marlin's cowboy rifles.All of mine are oversize by a surprising amount.
BigMike

P.S.
I measured another like mine that was just over .455".