1. The XM21 was a heavy barrel match M-14 rifle built by Rock Island Arsenal for the military. They originally build about 1,400 of them for use in Vietnam. In 1971, it became the M21, and was the primary US Army sniper rifle until 1988.
2. The M14E2 and XM21E2 rifle had the E2 walnut stock with a high butt, dropped comb and pistol grip. It was originally developed to give better recoil control in fully automatic fire, either without the heavy bipod of with one.
3. The ammunition loaded for the M60 machine gun was a larger diameter, .3086, to give good accuracy as the barrel heated and the bore diameter increased during sustained firing. It operates at 65,000 PSI, and was found to give excellent long range accuracy in the match bolt rifles of the 1960s, so it was issued to snipers for their bolt actions.
This is essentially the same 9-degree boattail FMJ bullet developed for the 1919 Browning machine gun in 1926, and used in the 1903A3 and 1903A4 sniper rifles of World War II.
You may be confused about the M80 ball ammunition because the standard M60 proof load for 7.62 NATO ammo had 41 grains of IMR-4475 behind a 172-gr. match-grade bullet producing 67,500 PSI. The same load was used in the M80 ball ammo but had the 148-gr. standard service bullet which produced 50,000 PSI.
