United Defense M42

United Defense M42
UD M42 submachine gun
Type	Submachine gun
Place of origin	United States
Service history
In service	1942-1945
Used by	See Users
Wars	World War II
Production history
Designer	Carl G. Swebilius
Manufacturer	United Defense Supply Corp.
Produced	1942–1943
No. built	15,000
Specifications
Weight	10 lbs., (4.1 kg)
Length	32.3 in. (820 mm)
Barrel length	11 in. (279 mm)
Caliber	9×19mm Parabellum .45 ACP (Prototype model)
Action	Blowback
Rate of fire	700 rpm
Muzzle velocity	1,100 ft/s (335.3 m/s)
Feed system	25-round box magazine (also issued with two 25-round magazines welded face-to-face)
Sights	fixed front post, rear adjustable for windage

The United Defense M42, sometimes known as the Marlin for the manufacturer, 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).

The M42 was developed by United Defense Supply Corporation specifically as a replacement for the Thompson submachine gun, which the U.S. military considered both expensive and complicated to produce. Made in both 9×19mm 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 World War II. Only six .45 ACP prototype test guns were made.

The weapon holds 25 9mm rounds in its magazine (designed by John E. Owsley, covered by patent 2,289,067). It has a cyclic rate of 700 rounds per minute. Frequently two 25-round magazines were welded face-to-face allowing a quick reload when the first became empty (see illustration). The weapon itself weighs 10 lb (4.54 kg)(empty), with a length of 32.3 in (820 mm). The barrel length is 11 in (279 mm), 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. 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. However, proving ground tests showed it was easier to strip and maintain than the Thompson or Sten Mark II and was more accurate at 100 yards. Despite its expense and precision, the UD42 enjoyed a good reputation in OSS usage.