M3 submachine gun

The M3 is what happens when an ordnance board stops asking for a good submachine gun and starts asking for a cheap one. Adopted in December 1942 to replace the Thompson, it was drawn by George Hyde around sheet steel and a car-parts factory rather than around a machine shop, and its receiver is two stamped halves welded into a tube. Soldiers called it the grease gun, after the tool it resembles.

Almost everything about it follows from the cost target. There is no semi-automatic position, because a selector is parts; the bolt is instead made deliberately heavy so that the cyclic rate falls to 450 rounds a minute, slow enough that a trained hand can release the trigger after one round. It fires from an open bolt using advanced primer ignition, so the cartridge is struck while the bolt is still travelling forward and part of the bolt’s momentum has to be reversed before the case can move — which is what allows a weapon this light to be simple blowback in a cartridge this large. The bolt is drilled through longitudinally for two guide rods carrying the return springs, a layout that tolerates loose machining because the rods, not the receiver walls, hold the bolt in line.

The original M3 is cocked by a crank on the right of the ejector housing. That mechanism was the first thing to break in service, and the M3A1 of late 1944 deleted it entirely: the bolt has a finger hole cut into its top face and is pulled back by hand through the open ejector port. The same weapon accepted a factory kit of barrel, bolt and magazine adapter that rechambered it for 9x19mm Parabellum, and about a thousand were built that way for the Office of Strategic Services, so that agents could be supplied from captured German stocks. The double-column, single-feed magazine was copied from the Sten and inherited the Sten’s sensitivity to dirt along with its shape.

It was issued as a disposable item — no unit-level spares, no armourer’s tools — and it outlasted that intention by half a century, riding in American tanks until the 1990s.

Gaps on this page: Guide Lamp’s empty magazine weight is illegible in the scanned manual and is not recorded here, and no free source publishes the M3’s width or height. The overall length is the 29.8 inches printed in FM 23-41; the encyclopedia gives 29.1 inches without saying which weapon or configuration it measured, and that figure is not used. Production totals and the 1943 unit cost come from an uncited infobox and are marked as estimates. The M3A1’s weight is the only figure recorded for that variant, because the manual’s change sheet describes the alterations without repeating the data table.