Steve, SUBmachine gun is not a legally defined term and as such is used in different places to mean different things. In the USofA:
Code of Federal Regulations Title 27 Chapter II Subchapter B Part 478 Subpart B Section 478.11
When used in this part and in forms prescribed under this part, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms shall have the meanings ascribed in this section. Words in the plural form shall include the singular, and vice versa, and words importing the masculine gender shall include the feminine. The terms "includes" and "including" do not exclude other things not enumerated which are in the same general class or are otherwise within the scope thereof.
Handgun.
(a) Any firearm which has a short stock and is designed to be held and fired by the use of a single hand; and
(b) Any combination of parts from which a firearm described in paragraph (a) can be assembled.
Pistol.
A weapon originally designed, made, and intended to fire a projectile (bullet) from one or more barrels when held in one hand, and having (a) a chamber(s) as an integral part(s) of, or permanently aligned with, the bore(s); and (b) a short stock designed to be gripped by one hand and at an angle to and extending below the line of the bore(s).
Revolver.
A projectile weapon, of the pistol type, having a breechloading chambered cylinder so arranged that the cocking of the hammer or movement of the trigger rotates it and brings the next cartridge in line with the barrel for firing.
Semiautomatic pistol.
Any repeating pistol which utilizes a portion of the energy of a firing cartridge to extract the fired cartridge case and chamber the next round, and which requires a separate pull of the trigger to fire each cartridge.
Machinegun
Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.
The following is NOT included and is not a LEGAL term
Submachine Gun
An air-cooled, magazine-fed, automatic carbine designed to fire pistol cartridges. The term "submachine gun" was coined by John T. Thompson, the inventor of the Thompson submachine gun. Thompson added the "sub-" prefix to denote the fact that his Machinegun used Pistol ammunition rather than Rifle ammunition. British Commonwealth sources often refer to SMGs as "machine carbines". Other sources refer to SMGs as "machine pistols" because they fire pistol-caliber ammunition, for example, the MP-40 and MP5, where "MP" stands for Maschinenpistole ("machine pistol" in German). However, the term "machine pistol" is also used to describe a handgun-style firearm capable of fully automatic or burst fire, such as the Stechkin, Beretta 93R and the H&K VP70. Personal Defence Weapons (PDW) such as the FN P90 and H&K MP7 are also commonly referred to as submachine guns. In addition, some compact rifles, such as the Colt XM177, HK53 and AKS-74U, are also referred to as SMGs, because they are used in the submachine gun role.
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