Commonly called the "Thompson."

The Thompson was very popular, but was heavy for a submachinegun, and many feel it was overly complicated, and expensive to manufacture.

John T. Thompson who helped develop the M1903 Springfield rifle and M1911 .45 caliber pistol, began work on a "trench broom" for close quarter combat shortly after his retirement from the Army in 1918. He recognised that the .45 caliber slug used in the M1911 pistol would be devastating when used in a fully automatic weapon.

By the spring of 1920 Thompson's company (Auto-Ordnance) produced a prototype capable of firing 800 rounds a minute.

Despite its excellent test performance, the Thompson was not adopted for use by either the US Army or Marine Corps. Still, Thompson contracted with Colt for the manufacture of 15,000 guns, designated "Thompson Submachine Gun, Model of 1921". The 15,000 guns manufactured by Colt lasted until the eve of World War II. In 1940, the U.S. Army ordered 20,000 
Thompson submachine guns; in 1941 the Army ordered an additional 319,000.

One of the main assets of the Thompson was reliability; it performed better than most submachine guns when exposed to dirt, mud and rain.

The main complaints against the Thompson were it's weight, inaccuracy at ranges over 50 yards, and lack of penetrating power.