When the Army decided to adopt the Heckler and Kock G28 as the Compact Semi-Automatic Sniper System to be the replacement for the M110, it seemed as if semi-automatic sniper rifles were a new thing.
Nope. In fact, during the Vietnam War, the Army was proving a semi-auto could be a very lethal sniper rifle. They took a number of M14 rifles — which were being phased out in favor of the M16 rifle — and added a scope.
I hope you like gun-speak — there are some sweet (technical) nothings coming your way...
The M14 had a lot going for it. It fired the 7.62x51mm NATO round — the same one as the M60 machine gun and the M40 sniper rifle the Marines used. But there were two huge differences between them that made the M21 a much more lethal rifle.
The M40 is a bolt-action rifle that carries five rounds in an internal magazine. The M14/M21 rifle is a semi-auto that has a 20-round detachable box magazine. The baseline M14 was deadly enough — Chuck Mawhinney is said to have used an M14 rifle to take out 16 of the enemy in one incident during the Vietnam War.
Once a Redfield scope (the three-to-nine power Adjustable Ranging Telescope) was added to the rifle and match-grade ammo was added to the M14, Army snipers had a real weapon.
Adelbert Waldron would use that rifle to become America's top sniper with 109 confirmed kills. For the record, that's 16 more kills than Marine Corps legend Carlos Hathcock has – and Hathcock was the third-best Marine sniper, behind Mawhinney (103 confirmed kills) and Eric England (98).
The M14 also saw action in Grenada for the Army. Inexplicably, after proving the M21 was an awesome system, the Army chose to go back to the outdated bolt-action with the M24.
But the War on Terror soon saw the M21 making a comeback. The M24 is still sticking around, now upgraded to the M2010 configuration, but was also slated to be replace by the M110.
In short, the Army has re-proven what they proved almost five decades ago — that a semi-auto rifle can be an awesome sniper rifle.