I've not seen that head stamp before. I looked it up and the best I could come up with is that it is a 7.62X51 (called .308 commercially) made at Lake City munitions plant in MO. The G3 might indicate this round was made to fire in a Heckler and Koch machine gun, but not positive. I surprised there is no date on the head stamp. Gary
The cartridge is Lake City Arsenal, 1963, MATCH means it was loaded for competitive shooting. Match grade ammo has precisely measured powder and bullet weight so the ballistics will be that same from one shot to the next. Being 1963 should make it 7.62 NATO since 30-06 was being phased out by then. You'd need to measure the length to be sure