OK, I got out my "small hole gauge". It's much easier to operate than I remembered and very repeatable. I measured a 1940 skeet barrel and a 1942 full barrel. Both bores are right on .410. The muzzle of the skeet barrel is .399 and the full barrel is .385. We're talking 11 thousandths for the skeet and twenty five thousandths for the full. .011 is a lot of choke for a modern .410 skeet gun. However, it was probably just right in 1940.