A fully automatic firearm - one that fires continually whilst the trigger is depressed - firing pistol ammunition. Indeed they're known as 'machine pistols' in several languages, although in English this term is usually reserved for the smaller models that could theoretically be operated with one hand.
Autofire is a key feature, although most models can also fire semi-automatic shots and/or limited bursts … coupled with the short range of pistol rounds this tends to put the submachine gun into the 'short range bullet hose' bracket where they compete with shotguns.
The submachine gun had a fairly short life as a military weapon - it evolved just in time for the end of The Great War as things like the Villar-Perosa and the Bergman MP-18 and was developed through the inter-war period, albeit with little interest from most armies.
During the course of WWII the SMG had a brief spurt of popularity, rising from being issued on the order of one per squad to being a weapon that armed entire regiments, but by the end of the war a replacement was already in sight - the Assault Rifle had more range, accuracy and hitting power for only a little more size and weight.
Today new SMG designs are pretty rare and users tend to be turning away from the cheap and nasty bullet hose (such as the Sten or PPSh) towards higher end weapons like the MP-5 family, generally used for building entry or close protection work where a high rate of fire and a limited ability to over-penetrate cover can be considered advantages.
Some of the more conservative interwar designs are easily mistaken for carbines … which could provide a nasty surprise for somebody.
Game and Story Use
- Cheap and nasty SMGs like the PPSh, M3 or Sten may not be great weapons, but they are damned easy to make and can usefully be placed in the hands of mooks. Their inherent inaccuracy and auto-only tendencies will help explain why all the PCs enemies seem to have attended the Stormtrooper Marksmanship Academy.