GO: Corresponds to the minimum chamber dimensions; If a rifle closes on a GO gage, the chamber will accept ammunition that is made to SAAMI maximum specifications
Headspace is measured from the bolt face to one of four locations, depending on the cartridge used
Headspace standards are set by SAAMI (Sporting Arms and Ammunition Manufacturers Institute) specific
Go Gage: Go Gages match the minimum chamber dimensions allowable under SAAMI specifications