I believe you're correct on the .338. However, I've seen plenty of photos and videos of .50 caliber Barretts, etc. being employed over there. How does it violate the Geneva Convention?
Common misconception. There is nothing in the laws of war about shooting enemies with a .50 cal. This whole concept is just one of those military legends. And, as I learned, don't shoot at the enemy: shoot at the web gear on his chest (it's OK to destroy enemy equipment).
The closest proscription is shooting them with something designed to cause unnecessary suffering (necessary suffering being undefined): The reason the .50 is used isn't for causing unnecessary suffering, it's to reach that far.
Just as the M118LR 7.62x51 sniper ammo has a hollow-point bullet. Not to expand or hurt more (anyway, if the shooter does his job it won't hurt at all), the hollow point is there for the accuracy of the bullet shot.