The issue of calibers larger than .50 is a serious one in US domestic law (NFA AOW). It is not a serious one in international law, at least not that I've ever heard of.
Furthermore, there are all kinds of military weapons greater than .50-caliber that are routinely used in anti-personnel applications.
Case in point: Aircraft. Almost all modern gun-equipped combat aircraft have 20 to 30mm cannons; .30 to .50-caliber machine guns are now the exception, not the rule. Do you think the pilots and gunners aboard such aircraft are expected not to fire on enemy ground troops? Hardly.