The MAC-10 is a highly compact, blowback operated

Blowback (arms)

Blowback is a system of operation for self-loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gases created by the ignition of the propellant charge....

 machine pistol

Machine pistol

A machine pistol is a handgun-style, often magazine-fed and self-loading firearm, capable of fully automatic or burst fire, and normally chambered for pistol cartridges. The term is a literal translation of Maschinenpistole, the German term for a hand-held automatic weapon...

 developed by Gordon B. Ingram in 1964.
The M-10 was built predominantly from steel stampings. A notched cocking handle protrudes from the top of the receiver

Receiver (firearms)

In firearms terminology, the receiver is the part of a firearm that houses the operating parts. The receiver usually contains the bolt carrier group, trigger group, and magazine port. In most handguns, the receiver, or frame, holds the magazine well or rotary magazine as well as the trigger mechanism...

, and by turning the handle 90° would lock the bolt, and act as an indicator that the weapon is unable to fire. The M-10 has a telescoping bolt

Telescoping bolt

A weapon with a telescoping bolt is one with a bolt which telescopes over, that is, wraps around and past, the breech end of the barrel...

, which wraps around the barrel. This allows a more compact weapon, and balances the weight of the weapon over the pistol grip where the magazine is located.