Actually, the terms "submachine gun" and :machine gun", though still in use, no longer serve as useful or descriptive classifications. Originally (circa WW I), a "submachine gun" was a shoulder fired, one man weapon, chambered for a pistol caliber round, and a "machine gun" was a crew-served, mounted weapon chambered for a rifle-caliber round.

But even before the end of WW I, the search for heavier firepower in highly mobile weapons began to blur the lines between submachine guns and machine guns. One of the first was the Browning BAR which was a shoulder fired, one man weapon chambered for a rifle caliber round, and capable of firing armor piercing bullets. Then came air-cooled, shoulder fired, bipod-mounted machine guns chambered for rifle-caliber rounds and
capable of being picked and fired on the move or used in static defensive positions. The development of "intermediate" (between rifle caliber and pistol caliber) cartridges further blurred the distinction since these combined the capabilities and advantages of the submachine gun and light machine gun without supplanting either.

The heavy machine gun (either air or water cooled) retained some important capabilities, such as sustained heavy fire and large caliber hitting power, which couldn't be built into either light machine guns or submachine guns, and thus remained in the arsenals of almost all armies. Today, machine guns and submachine guns are not strictly divided as they once were, and there is a whole spectrum of weapons from very small .32 ACP caliber submachine guns up to very large, vehicle mounted, multi-barrel, 30 MM machine guns, with no easily defined dividing line between them.