SERVICE – US Intelligence Services and Special Operations Forces
The M3 suppressor was the first such device developed by the US Army. It was part of an Army Infantry Board program to decide if a suppressor would be useful for sniping weapons. The bulk of the suppressors tested were made by Bell Laboratories with one design, a suppressed Thompson submachinegun, being from the Chrysler Corporation. Most of the suppressor designs were not considered acceptable except for the Bell Labs design for the M3 submachinegun. Though the suppressor design was considered successful, it was not considered particularly useful by the Infantry Board. It was picked up very quickly by the new US Intelligence service, the Office of Strategic Services (OSS) who ordered 1,000 of the suppressors with an additional order for 4,000 being issued before the end of World War II.
The M3 suppressor is a two-stage design. The first stage is the expansion chamber that, in this case, surrounds the barrel. It does the initial slowing and cooling of the propellant gases, which are the primary source of noise in a gunshot. In the case of the M3 design, the first stage completely surrounds the barrel of the weapon for its length. The barrel is ported by a series of four rows of holes that extend the length of the barrel. Each row being twelve 6.35mm (¼ inch) holes, making for 48 holes in total. This portion of the barrel is surrounded by the rear tube of the suppressor which is 19 cm (7.5 in) long and has a diameter of 4.1 cm (1.63 in). Contained within the rear tube, and surrounding the barrel, is a roll of 16-mesh wire screen that completely fills the space between the outer tube and the barrel. The screen does the initial job of slowing and cooling the propellant gases that bleed through the 48 holes in the barrel.
The end of the barrel itself is threaded to accept an adapter plug that is itself threaded to accept the insertion of the front tube assembly that acts as the muffler portion of the device. The adapter plug also completely closes off the front of the rear tube, leaving only the bore of the device for the propellant gases to escape.
To further slow and cool the propellant gases is the front tube, or diffuser end, the second stage of the suppressor. It is a steel tube 16.1 cm (6.36 in) long and 2.9 cm (1.13 in) in diameter that threads into the adapter plug. Inside of the front tube is a stack of about 230 wire mesh washers. The washers are each made of 30-mesh metal screen about 2.5 cm (1 inch) in diameter with a 12.7mm (1/2 inch) hole in the center. The screen baffle stack is held in place by the threaded muzzle cap which also has a 12.7 mm (1/2 inch) hole in its center
The suppressed M3 was considered second to the suppressed British Mk IIS Sten gun during testing. Part of this listing is due to the noise of the M3 action, which was considered by many observers to be the loudest sound of shooting the weapon. The actual sound of the gun’s discharge was considered a mild clap of the hands, the Sten that of a loud hiss. The M3 suppressor did require cleaning and brushes were issued with the weapon for the tube and the gas port holes in the barrel. At least a partial cleaning was considered necessary every 200 rounds. Wire screen rolls and washers were available for replacement in the weapon, along with an alignment rod for maintaining concentricity of the washer stack. The original dirty screen roll and washer could be cleaned a reused, though it was warned that there should be no oil on the parts when assembled. Oil on the screen would burn and smoke, giving a telltale smell and observable cloud. The rod, held within the diffuser tube, and brushes, were packed with the M3 suppressor when it was issued.
