Gauge, Breech Bore Taper, Cal .30, C-3940. Used to measure the throat erosion. Browning M1919, M1917 and M37 machine gun barrels. Graduated in .001 increments. Wide reject grooves for quick field serviceability tests. Includes adapter ring to measure 7.62mm M1917, M1919 and M37 barrels. Simply slide the ring over the gauge and read at the edge of the gauge. New.