This ZPV Headspace Gauge Field Set is necessary when finish chambering a barrel or when checking the chamber of a used gun to be sure the headspace dimensions are within safe limits. A Go Gauge and a No Go Gauge are required to properly headspace a barrel. Field gauges are used to determine if a gun is safe to fire.Field gauge (slightly longer than "NO-GO"), is used in the field to indicate the absolute maximum safe headspace. Field gauge is 0.1mm(0,004inch) longer than a NO-GO gauge.Go Gauges represent the lower limit of SAAMI specifications for proper headspace dimensions. If the action closes with this gauge is inserted, then the firearm is within acceptable minimum headspace dimensions.No Go Gauges represent the upper limit of SAAMI specifications for proper headspace dimensions. If the action closes with this gauge inserted, then the firearm may not be within acceptable maximum headspace dimensions and should be examined further by a knowledgeable gunsmith. FeaturesSet includes...