A submachine gun or SMG is a select-fire weapon, either handheld (machine pistols) or shoulder-fired, that fires pistol-caliber rounds. Its primary role was that of a close-quarters weapon; it has since been replaced by short-barreled rifles (carbines) for this purpose.
The concept of the submachine gun dates back to World War One; the trench warfare of this period required effective, compact weapons for close combat in trenches; additionally, a lightweight and maneuverable fully automatic weapon was desirable to complement light machine guns in both defensive and offensive scenarios.
The first pistol-calibre machine gun was the Italian Villar Perosa, which was originally designed as a lightweight, portable gun for mobile bicycle infantry units. During World War I, the Italian Army realized its potential as a trench-clearing weapon, and attempts were made to repurpose it into an infantry carbine, the resultant weapon being the Beretta M1918.
The first submachine gun to see widespread infantry use was the German Bergmann MP18.I. Designed specifically for trench combat, the MP 18 was fielded by German stormtroopers in 1918 and proved effective enough for the concept to be investigated further after the war ended.
In the inter-war period, a variety of new submachine gun designs began to appear. The most prolific was the Thompson, commonly known as the "Tommy gun", which became notorious for its use by criminals and gangsters in the prohibition-era United States. Other successful designs included the German Haenel MP28.II, the Austrian Steyr-Solothurn S1-100, and the Finnish Suomi KP/-31, which all saw military adoption by their respective countries of origin.
World War II marked the zenith of the submachine gun, with German troops utilizing the MP 40 submachine gun to great effectiveness. In response, the militaries of the UK and the USSR mass-produced cheap submachine guns like the Sten and PPSh-41 as emergency measures. The US Army also substantially increased its arsenal of submachine guns during the war, issuing large numbers of not only Thompson guns but also M3 Grease Guns.
During the war, submachine guns were typically issued to officers and high-ranking NCOs. However, on the Eastern Front, submachine guns became increasingly common, with some entire battalions of the Red Army being issued with PPSh-41s. During the hectic close-quarters combat that typified battles such as Stalingrad, submachine guns became sought after for their inherent advantage over slower-firing bolt-action and self-loading rifles.
In 1944, Germany began issuing the StG 44, which offered higher stopping power, better accuracy, and increased range over submachine guns. Although the StG 44 came too late in the war to make much of a difference, it was studied after the war ended by various countries and inspired an entirely new classification of weapon; the assault rifle, which was based on the same principles as the submachine gun but chambered for an intermediate rifle cartridge. Soon rifles like the Russian AK-47 and American M16 became standard-issue and made both conventional rifles and submachine guns somewhat obsolete. As such, the issue of submachine guns in most militaries became increasingly uncommon and they were phased out in favor of assault rifles.
The submachine gun gained a new lease of life during the 1970s and 80s, when it became the preferred weapon of special forces units due to its compact size and effective close-quarters capability. Guns like the German-made Heckler & Koch MP5, Israeli Uzi, and American MAC-10 were common weapons of choice for elite military and counter-terrorist operatives.
Today, submachine guns are an increasingly rare type of weapon, with relatively few new designs being produced. A similar type of weapon known as the personal defense weapon has been developed for effectively the same purpose. Very few military forces still issue large numbers of submachine guns to their troops.
The submachine gun was once used as a CQC (close-quarters combat) weapon, usually in house and room clearings (owing to their usually compact size). Due to the fact that pistol calibers are poor terminal performers and carbines packed much more power in a similarly sized package, most law enforcement agencies, as well as the military, became increasingly reliant on the carbine to take over the role of the SMG.
However, the submachine gun still holds one advantage: when fired suppressed, the SMG is quieter than a carbine.