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One of the many changes that emerged during the First World War was a diversification of weapons; the submachine gun being an example. It was handy, offered a high rate of fire and was based on the blowback operating system. 

Blowback systems had evolved just before the turn of the 20th century and was used in a number of pistol designs. During firing, with this system, the breech remains unlocked and relies upon the mass of the breechblock and the strength of the return spring to support the cartridge case until the pressure has dropped to safe levels. It is simple and cheap to manufacture but can only handle low powered cartridges.

In 1915 the German army wanted a weapon that could sweep trenches once the enemy parapet had been gained. Hugo Schmiesser, who had designed several blowback based pistols for Bergmann, put the principle to use in a carbine sized weapon that fired a 9mm cartridge from a 20-32 round detachable magazine. The weapon had a conventional wooden stock, a barrel housed in a perforated jacket, a bolt and a simple trigger group. Insert a preloaded magazine and it was ready to fire. Its name was the Bergmann Machinenpistole 18I or MP18I and it delivered withering close-in firepower. 

Development began in 191630,000 being manufactured by 1918it was one of the favorite weapons of the new Stosstrupps and Gruppen who used new infiltration tactics on the Western Front in 1918. 

Seeing a need for the same type of weapon the US Army designed and manufactured the very innovative Pedersen Device which allowed, with slight modification, a standard Springfield rifle to be turned into a submachine gun firing the low powered .30 Caliber, M1918 cartridge. But the war ended before they could be put to use. 

There are a myriad of designs for submachine guns but they all have the following common elements i.e. they almost most always rely on the blowback system, have a plastic, metal or wooden stock, a short barrel, a heavy bolt and elementary trigger mechanism, use cheap pressed steel parts and are chambered for pistol calibers that range from .30 to .45 caliber.

In the interwar years the Thompson submachine gun (.45 cal) was used by the marines in Nicaragua and by the U.S. Coast Guard. A simplified version (M1 Thompson) was used by the thousands in Europe and the Pacific. At the same time German Panzer troops wanted a weapon that was small enough to fit into an armored vehicle, while providing the crew with high personal defensive fire power: A combination that also appealed to airborne troops. The Germans provided their units with the all metal MP38 (9mm) and later the MP40 (9mm). The latter used more stamped parts and was cheaper to manufacture.

British troops were given the Sten gun and the Russians after tasting the sting of the Finish Soumi submachine gun during the Winter War embarked on the manufacture of millions of the PPD40, PPSh41, the PPS42 and 43, firing the 7.62mm Soviet pistol cartridge. This weapon met two criteria. Tactically it fit Russian doctrine to close with and mix it up with the enemy and the extensive use of pressed metal meant that they could be quickly and cheaply manufactured by small metal shops.

After the war assault rifles and assault carbines displaced the military use of this weapon. Weapons such as the Uzi and the Heckler and Koch family of submachine guns are almost exclusively used by police forces though sometimes in new calibers such as the .40 S&W and 10mm. -- Peter38