45 ACP versions, as most loads are subsonic already, as opposed to special, low-powered subsonic loads usually required for suppressed 9mm weapons. The barrel is threaded to accept a suppressor, which works by reducing the discharge's sound without attempting to reduce the speed of the bullet. In addition, this design incorporates a built in feed ramp as part of the trigger guard (a new concept at the time) and, to save on cost, the magazine was recycled from the M3 Grease Gun. The MAC-10 fires from an open bolt, and the light weight of the bolt results in a rapid rate of fire. This allows a more compact weapon and balances the weight of the weapon over the pistol grip, where the magazine is located. The MAC-10 has a telescoping bolt, which wraps around the rear face of the barrel.

A notched cocking handle protrudes from the top of the receiver, and turning the handle 90°, locks the bolt, and acts as an indicator the weapon is unable to fire. The MAC-10 is built predominantly from steel stampings.

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