A functional copy of the "Gauge, Breech Bore Taper, Cal .30 C-3940" used to measure the throat erosion on 1903 Springfield type rifles and Browning 1919 and 1917 machine gun barrels. Graduated in .001 increments, read these as you would any other throat erosion gauge. Wide reject grooves for quick field serviceability tests. Laser engraved for crisp, precise markings. Made in Ohio by us at Ravenna Armory.