Muzzle Erosion Gauge for M1 Garands and other 30 cal rifle bores. Brand new and made in the USA. Zero ("0") line on gauge is .300 inch diameter (30 cal ) with graduation lines every .001 inches. Gauge will measure bore diameter from a range of .298 ("-2") to .304 ("4") inches. These will work for gauging any 30 caliber muzzle.