The thread for the Hornady overall length gauge is 5/16-36 UNS form memory. I have a tap at work and we made a special lathe chuck which centres the case mouth and grips the case at the base. It works really well and we machine quite a lot of cases for other people, especially on fire formed cases. To open the neck you can wind the sizing button right down and carefully push it through the neck only and then pull it straight back out without letting the main body touch the neck.

Waidmannsheil.