I would assume that Model 42 bores are all pretty much the same, but measuring .410 bores accurately is not easy with the tools we normally have access to. The most accurate I have found with normal machinist's tools is a "small hole gauge" inserted into the bore, screwed tight, and then driven out the breech and measured with a micrometer. Unfortunately, it is not easy. I have not used a .410 bore micrometer, but I assume they are also quite accurate. Have any two posters on this thread come up with the same bore diameter on a 42?
