410 is not that difficult to measure with a telescoping gauge and a micrometer, and Bill claims to have done so.
With my El Cheapo Chinese $20 set I can get .0005 resolution.
The trick to measuring the bore behind the choke is to insert from the muzzle end, lock the gauge, and carefully slide it down the bore and out the breech end. A small rolled up paper tube will assist in keeping it straight. It's tedious, but can be done.
My K-20 measures .4015, which apparently is the norm for Germany. The chokes measure as expected relative to the bore.
Still.... 50 thou is a lotta choke but we don't know the actual constriction until we measure the bore.
