The United Defense M42, sometimes known as the Marlin for the company that did the actual manufacturing, was an American submachine gun used during World War II.

It was produced from 1942 to 1943 by United Defense Supply Corp. for possible issue as a replacement for the Thompson submachine gun and was used by Office of Strategic Services (OSS) agents.

[1] However, its usage was limited, and the Thompson continued to see service until the end of the war, alongside the M3 submachine gun, which was designed around the same time as the M42.

They were also strongly advised to keep the double magazine clear of grass, mud, dirt and any other debris out because of the risk of jamming.

However, proving ground tests showed it was easier to field strip and maintain than the Thompson or Sten Mark II and was more accurate at 100 yards (91 m).

At least 4,000 sent to United States Navy installations in the Far East were transferred to Dai Li's regular resistance forces in China for use against the Japanese invasion; and some later equipped Communist Chinese soldiers.

[citation needed] Overall the weapon failed in its intended role (to replace the Thompson) but proved effective in limited use in the hands of resistance forces.