m3 submachine gun
The M3 "Grease Gun" was .45-caliber weapon introduced during World War II in an effort by the U.S. Army to design a less expensive and faster-to-produce replacement for the Thompson submachine guns, using the stamped-metal process of the German mp40 as inspiration. It got its nickname because of its resemblance to a grease applicator.
Early versions were plagued by defects and failures due to loose tolerances during the manufacturing process, in particular the reliance on a single weld of the barrel shroud to hold the internals in place could (and often did) lead to the weapon literally disassembling itself on the battlefield, when the vibrations of the weapon firing caused a failure of that weld and sent the internal mechanisms flying downrange. These and other issues with early models led to great distrust of the Grease Gun by many American soldiers, with some refusing to part with their Thompsons or even refusing to equip Grease Guns that were issued. Despite this, an improved pattern of the Grease Gun (M3A1) debuting in 1944 had solved most of the weapon's issues, and the infamy from the earlier pattern did not last after the end of WW2.
Identifying details include a straight magazine well/foregrip, barrel with no sight post, cylindrical body, and retractable wire buttstock.