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Silent but Deadly: The Suppressed M3 M3A1

By Michael Heidler

The successful use of the silenced Sten Mk II S for the British Special Operations Executive (SOE) also aroused the U.S. Ordnance Department‘s interest in a suppressor for SMGs. Founded in June 1942, the Office of Strategic Services (OSS) could use such a weapon for it’s own special operations and to support resistance groups in Europe and the Pacific Theatre. The M3 seemed to be well suited for that purpose.

Standard.45 ACP rounds fired from short barreled submachine guns are inherently subsonic, it is one of the most powerful pistol calibers available for use in suppressed weapons. Suppressors reduce the audible report by slowing down and channelling the high speed gas generated by the burning and expanding gunpowder before it exits the muzzle. Suppressors cannot act on the supersonic shock wave continuously generated by a bullet exceeding the 1,084 ft/s (332 m/s) speed of sound at 32 °F (0 °C) ambient temperatures. This shock wave is continuously produced throughout the entire flight path over which the bullet is supersonic, which extends long after it exits the barrel. One major disadvantage of automatic and weapons, however, remains: the unavoidable noise of the moving internal parts when the spent case gets extracted and the next round gets automatically loaded. So a weapon with suppressor is not completely silent, but it lacks the typical hard “bang“ and for the enemy it is more difficult to locate the direction of the sound’s source.

The first trials with suppressed M3 submachine guns were already being held in 1942 at Fort Benning (Georgia). These devices were based on the designs of Hiram Percy Maxim, with curved vanes of thin sheet metal, but were not convincing. The design was heavy, the gun became extremely top-heavy and the suppressor heated up quickly during continuous fire. Two different models were tested, a long one with 20 inch barrel and a shorter one with 12 inch barrel. Because of the unsatisfactory results, the tests were discontinued.

In 1943, the Ordnance Department contacted the company Bell Laboratories on behalf of the OSS concerning the development of...


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