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Technically, the military uses the .50cal rifles against equipment, not personnel (helicopters, light vehicles, radar, etc.). It is against the Law of Armed Conflict to deliberately use .50cal ammo against personnel. I'm certain that the U.S. military would never do that... 
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I was told at the two-week Army Law of War Seminar (at their JAG School at UVA) that this is an old wive's tale, and even as a JAG with a significant amount of LOAC education and field experience I can't see any reason the use of a .50 cal with ball ammo would be illegal. See, for example,
this Navy JAG page, which plainly states that "use of .50 caliber weapons against individual enemy combatants does not constitute a violation of [the] proscription against unnecessary suffering or superfluous injury." Also, see the USMC's take on this,
here:
Caliber .50 weapons.
1. There is a common misperception that the .50 caliber machinegun may not be used against personnel. It was effective as a long-range sniper weapon in Vietnam and Korea.
2. FM 2365, Browning Machinegun Caliber .50 HBMG (1972), is
consistent with the law of war in authorizing use of the .50 caliber MG against enemy personnel.
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It's against those same laws to use shotguns in war, as well. And I saw several Marines slinging Remingtons over their shoulders on CNN. >>
This is not true either. Shotguns are not prohibited by the Hague Convention or any other doctrine of the Law of War.