I prefer to take a fire formed spent casing, take a triangle jeweler file and file a hairline slit in the length of the neck, pinch slightly for to provide adequate friction on the bullet, insert a bullet of your choice in the neck, chamber the inert cartridge VERY SLOWLY AND CAREFULLY, pull the bolt back while making sure the bullet doesnt contact anything on its way out, mic the cartridge from case head to ogive.

Sorry for the long explanation but ive never had consistent readings from the various "tools" sold to make this this measurement. The combination of plastic and aluminum on the commercial OAL gauges leaves quite a bit to be desired. My method doesnt cost anything (assuming you have jeweler files)

Let me know if this makes sense to you. I can try to explain it better if needed.