The guys at Forgotten Weapons
have done a great video which explains small caliber machinegun terminology: submachine guns, personal defense weapons, and machine pistols. Terminology. Forgotten Weapons
tells us who makes them, why they exist and their place in military history.
Submachine Gun: Pistol caliber, fully automatic, and fitted with a shoulder stock. For example, Thompson, MP40, MAS-38.
Machine pistol: Handgun form factor and fully automatic. For example, Glock 18, Mauser Schnellfeuer, Stechkin.
Automatic Rifle: Shoulder or hip fired, limited magazine capacity, minimal sustained fire capacity. Examples: M1918 BAR, Chauchat.
Persinal Defense Weapon: (1) Armor-piercing or (2) holsterable submachine gun, not intended for front line combat. For example, FN P90, H&K MP7, Czech vz.61 Skorpion, Polish PM63 Rak.
Photo courtesy of US Marines. A Marine of the 1st Marine Division draws a bead on a Japanese sniper with his tommy-gun as his companion ducks for cover. The division is working to take Wana Ridge before the town of Shuri. Okinawa, 1945. S.Sgt. Walter F. Kleine. (Marine Corps)