Yes, you can aim at enemy troops with the .50-cal.
It's one of the most persistent myths in the U.S. Military. I was even told it in basic training a mere 11 years ago, almost 90 years after the .50-caliber M2 was first designed. It goes like this: Weapons firing a .50-caliber round can be aimed at equipment, but not people. So, if you need to kill a person with a .50-cal., you have to aim at their load-bearing equipment (basically their suspenders).
Look at this. War crimes at night. What is wrong with troops today?
(U.S. Army 1st Lt. Robert Barney)
But, uh, really? The U.S. has and deploys a weapon in an anti-personnel role that can't legally be fired at people? And we've just been hoodwinking everyone for a century?
That's... surprising, if not unbelievable. That would require that every enemy in World War II never brought war crimes charges against the U.S. If you assume that the rule was put in place after World War II, when a lot of modern war crimes were defined, then you still have to assume that no one in Korea, Vietnam, Panama, Iraq, or Afghanistan protested the illegal American actions.
And, even more odd, militaries brag about their top ranged sniper kills. Five of the top six longest-range kills, at least according to Wikipedia right now, were made with .50-cal. rounds (Number six was made by Carlos Hathcock with a machine gun, because he's awesome). Since all of those snipers were targeting individuals, if you accept this premise, aren't they war criminals?
Extremely accurate war crimes, huh, buddy?
(U.S. Marine Corps Sgt. Conner Robbins)
Uncool, Wiki editors — do not list war crimes made by war heroes. Let the military justice system do its work without your amateur meddling...
...Except, hear that? That's the sound of no CID agents coming to arrest these overly bold war criminals. Probably because shooting an enemy combatant with a .50-cal. is not, at all, illegal.
The actual rules for weapons in combat ban specific categories of weapons, like poisonous gasses or plastic landmines, and weapons that cause more unnecessary suffering than they provide military advantage.
If that sounds vague, that's because it is. Nations occasionally argue about what weapons cause unnecessary suffering, but the militaries involved would typically rather keep all their options open, and so combatants usually decide that any given weapon is fine.
Look at this guy and his belt-fed war crimes. Horrible.
(U.S. Army Spc. Deomontez Duncan)
Shotguns came under some serious contention in World War I. The U.S. brought them over the Atlantic to clear German trenches, and they were ridiculously effective. Germany complained that the weapons, which often left their troops either blown in half or with pellet-filled guts, caused unnecessary suffering. America just pointed out that Germany was already using poisonous gasses, and so they should screw off.
Germany never lodged a formal case against the shotgun, but there are a number of weapons that are, officially, illegal under rules against unnecessary suffering. Weapons that use plastic fragments or pellets to wound and kill the enemy, many types of landmines, some types of torpedoes, etc., have all either been banned or partially banned. But there's no real case against the .50-cal.
So, how did this misinformation campaign get started? It's not completely clear, but there is a rumor it began in Vietnam.
American logistics at the time were limited, especially for troops deep in the jungle. As the story goes, troops far forward were using their .50-cal. rounds to shoot at any and everything in the jungle that sounded threatening. Commanders prevented ammo shortages by ordering their men to use the .50-cal. ammo only to engage light vehicles.
This is the target that the .50-cal. is best for. It can pierce light armor at decent ranges unlike 5.56mm or 7.62mm rounds. So, if you have a limited supply of the ammo, you want to hold it for the vehicles. The command is thought to have grown from simple ammo conservation to belief of a war crime.
But no, if it's an enemy combatant, you can legally kill it with any weapon at your disposal, as long as you don't damage civilian structures or intentionally cause undue suffering. You don't need to aim a .50-cal at their suspenders, belt buckle, or buttons.