M3A1 Grease Gun Submachine Gun
The United States military started to believe in the value of the mostly-defensive weapon of submachine gun after a successful British Sten Mark series and the famous German MP40 Submachine gun. The M3A1 Grease Gun first saw action in the European Theater during World War II, though not to much fanfare.
The shortcomings of this simply produced submachine gun became apparent in battle field conditions and soldiers were in favor of using the M1 Thompson "Tommy Gun", or even a German MP38 or MP40 if they found one. Even the .30 caliber M-1 Carbine was more appealing for a soldier. Nevertheless, the M3 Grease Gun was pressed into service and was used by many soldiers throughout the war.
In 1944 the Grease Gun was produced in an even simpler, but more effective way. It jammed less but was still not the most appealing weapon. The U.S. soldiers never fully accepted the Grease Gun even though 700,000 were made in the United States alone.