.50 cal falls under the definition of arms. It isn't until you break into the 1.0 cal that the definition becomes grey. We folk in the military classify all arms as ordnance, but only for supply purposes. The term is accepted as artillery or delivered weaponry.
It's all about definitions. Let's use a term that practically everyone in the world can agree upon: What is an "arms dealer" and what does he deal in? Small arms only, or does he also sell rockets, grenades, airplanes and bombs?