A .50 is small arms.
Nope. In international law they're light weapons. From the Wiki entry:
In international arms control, small arms include revolvers and pistols, rifles and carbines, assault rifles, submachine guns and light machine guns. Together with light weapons (heavy machine guns; hand-held grenade launchers; portable anti-aircraft and anti-tank guns; recoilless rifles; portable launchers of anti-aircraft and anti-tank missile systems; and mortars of calibres of less than 100 mm), they comprise the Small Arms and Light Weapons (SALW) protocol.