Correct firing pin protrusion is critical to the entire ignition sequence. Elliptical face on the recording plunger allows accurate measuring of both rimfire and centerfire pin protrusion to within .0001". To Use: Unlock plunger, place gauge over protruding firing pin, lock plunger, lift from bolt and mike overall length. Difference between original length and miked overall length of body plus plunger, is amount of pin protrusion. Mfg: Brownells SPECS: 1/2" (12.7mm) dia., .90" (23mm) lg. Hardened plunger.