The United Defense M42 was an American submachine gun in World War II. It was produced from 1942 to 1943 by United Defense Supply Corp. (a government-formed company specifically tasked with weapons development) for possible issue as a replacement for the Thompson submachine gun and was used by agents of the Office of Strategic Services (OSS).
Made in both 9x19mm Parabellum and .45 ACP prototypes, the 9 mm version was the only one to ever see widespread production. Manufactured by High Standard Firearms and Marlin Firearms, about 15,000 were produced in the last three years of WWII. Only six .45 ACP prototype test guns were made.
The weapon holds twenty 9 mm rounds in its magazine, and can fire them at 700 rounds per minute. The weapon itself weighs (empty), with a length of . The barrel length is , and it has six-groove right-hand rifling.
An extremely simple design, it was a straight blowback, selective fire weapon. It was built under "hurry-up" war conditions and some of its design flaws stem from this approach. Intended for use by U.S. troops at the time of its design, it found more favor being air-dropped to partisan forces in occupied Europe. Some of them were transferred to Dai Li's regular resistance forces in China to against Japan invasion. The use of the 9 mm caliber allowed resistance forces to use captured ammunition in their weapons, eliminating the need for repeated re-supply drops.
Problems with the weapon were varied. Under combat conditions it was found that the sheet metal magazines had a tendency to warp out of shape causing feeding problems. They had little tolerance for exposure to large amounts of mud and sand and tended to jam if not cleaned regularly. The gun was also labor intensive to produce. It used all machined parts, no stampings, and under wartime conditions machine work is at a premium.
Overall the weapon failed in its intended role (to replace the Thompson) but proved effective in the hands of resistance forces.