Ordnance Notes -- by Bob Stoner GMCM (SW) Ret.

M3A1 .45 Submachine Gun


The submachine gun, caliber .45, M3 and M3A1 began life in 1943 as successors to the Thompson submachine guns, M1928A1, M1, and M1A1. The problem with the Thompson was that it was too well made and required far too much material, time, and skill to manufacture even in its final simplified form -- the M1A1.

Guide Lamp Division of General Motors produced the M3 and further simplified M3A1. Because the M3 was so crude in appearance, the troops called it the "grease gun." The name has stuck ever since.

The photo shows the M3 model with its stock retracted. Note the prominent cocking lever ahead of the trigger. This is the main recognition feature of the M3. This particular M3 has been fitted with an M3A1 wire stock to help load its magazines. (Photo: US Army)

The M3 submachine gun is of all metal construction, primarily of stampings that are welded together to form the gun. The only machined parts are the extractor, barrel, and bolt. The stock is formed wire that retracts for added compactness. The safety is engaged by closing the ejection port cover (bolt open or bolt closed) and disengaged by opening the ejection port cover. The sights are a stamping welded to the rear of the receiver (rear sight) and a welded stamping (front sight). The gun is cocked for firing by means of a spring loaded lever to the rear of the magazine catch on the magazine well. The Office of Strategic Services (OSS, the father of today's CIA) made up a kit that converted the M3 to German 9mm caliber for use by European resistance fighters. The OSS also developed a very effective suppressor (silencer) for the gun.

The M3A1 is a further refinement and simplification of the basic M3. The M3A1 incorporates a larger oiler in the pistol grip. The cocking lever is eliminated for an enlarged ejection port with a cocking cut milled into the bolt itself. A guard is added to the magazine catch to prevent accidental dropping of the magazine. The stock is modified to act as a cleaning rod and magazine filling tool.

The M3A1 .45 submachine gun with its ejection port cover open. The plate riveted to the inside of the cover (a similar plate is used on the shorter M3 cover) is the safety for the weapon. A tang on the plate locks the bolt from moving when it is forward (mates with a hole at rear of bolt) or cocked (mates with a notch at front of bolt). On the M3A1, the front notch also serves as the cocking mechanism for the gun; you put your finger there and pull the bolt back to cock the piece. The bent tab welded to the underside of the wire stock is a magazine loader to help insert the .45 ammunition into the magazine because magazine springs were very stiff. (Photo: US Army)

The M3-series guns fire from an open bolt. That is, the trigger is pulled; the bolt goes forward and strips a round from the magazine; the bolt chambers the round in the barrel; the extractor snaps over the cartridge; the fixed firing pin fires the cartridge; the bolt recoils to extract, eject, and start another cycle. The gun continues to cycle as long as it has ammunition or the trigger is held down.

The M3 or M3A1 weighs about 8 pounds loaded. It is 23 inches long with the stock retracted and 30 inches long with the stock extended. It has a cyclic rate of 350 to 450 rounds per minute. The gun's maximum effective rage is 50 to 150 yards. The low cyclic rate of the gun allows very accurate placement of shots and “double taps" are very easily done.

The gun has been replaced in Naval Special Warfare units by the German H&K MP5-series of 9mm submachine guns.

© 2005 Bob Stoner R3