Article 23 of Section 4, Law of land warfare, Hague 1907.
It is forbidden to:
- To employ poison or poisoned weapons;
- To kill or wound treacherously individuals belonging to the hostile nation or army;
- To kill or wound an enemy who, having laid down his arms, or having no longer means of defence, has surrendered at discretion;
- To declare that no quarter will be given;
- To employ arms, projectiles, or material calculated to cause unnecessary suffering;
This "calculated to cause" language allows us to use Boat Tail Hollow Point bullets for sniper rifles. The bullets are designed to fly true and maintain high accuracy, not to cause unnecessary suffering.
The intent behind the bullet design is the legal issue, not the caliber. I'm not sure if there has been a JAG determination memorandum on 50 Cal API rounds, but I don't know if it matters as anything a 50 BMG sends downrange is going to smack things with authority.