I was told in Basic Training, that according to the Geneva Conventions, it is illegal to use .50-cal rounds on personnel, but only on equipment. Now, of course, canteens could be considered to be equipment...
I think it is totally ridiculous. All these rules make me angry, as though warfare is some sort of game like football. You're supposed to do what it takes to win. So what if my weapon inflicts ghastly injuries?
It was an effort to make war more 'humane'. What we learned later is it is better to wound your opponent than take him out. You don't have to be fastidious with the dead, but a wounded combatant can be 'saved'.
And yes, canteens are 'equipment' so using the 50 on it is allowed. Of course, if a wounded combatant poses a threat (still breathing) I think it would be prudent to eliminate the threat.