I measured six Model 42 skeet barrels with an inside measuring dial caliper and got .398, .398, .400, .400, .400, .402. I measured one full choke barrel and got .382. I am not sure whether the caliper is accurate to the thousandth on inside measurements, but I thought it was interesting that the difference between skeet and full is between 16 and 20 thousandths. There also seems to be considerable choke in the skeet barrel. In the days of paper shells and fiber wads, quite a bit of choke was apparently neccesary to reliably break skeet birds. I did not measure bore diameters of these barrels.
