During WWII the army was looking for a replacement for the Thompson submachine gun. The Army wanted a gun that was cheap to manufacture, easy to use and when it broke could just be thrown away.  They got the M-3 Grease gun.

It was originally manufactured in 45 caliber. The Office of Special Services (OSS) got wind of the gun and ordered it in 9 mm. OSS also ordered a replacement barrel in .45 caliber with a suppressor. It was described and offered in the OSS July 1944 equipment catalog.

The gun saw service relatively late in the war and was used by US and allied forces, including partisans.

A gun that was designed to thrown away when it broke, was still in the United States inventory and was issued to armored vehicle crews during the first Gulf War in the early 90’s.  Not bad for 20 bucks.