I know for the 12 gauge it's .806 to .808 or so. Right? What is it for the 410? I tried already to Google it and find it in my Lyman and RCBS shotgun manuals but no luck. If I had a box of new shells I'd measure them to see but I do not. I got the Sizemaster giving me .468 at the top of the metal and .474 near the base. Every once in a while one will not want to go into battery and I have to slam the break barrel gun shut to get it to close. Like one shell per box and I can feel on maybe 6 others that I barely gave enough force to close on them.