Slamfire

Slamfire occurs when the cartridge discharges as soon as it reaches the chamber, rather than waiting in the chamber to receive a firing pin impact when the trigger is pulled.
Schematic of an Advanced Primer Ignition blowback operation that works in a similar way to slamfire by striking the cartridge as its moving forward before it is fully chambered.

A slamfire is a discharge of a firearm occurring as a cartridge is being loaded into the chamber. Some firearms are designed to slamfire, but the term also describes a malfunction of self-loading firearms. Shooters accustomed to firearms requiring trigger activation for discharge may be unprepared for a slamfire discharge.

Some military firearms are designed to fire from an open bolt condition to avoid the unintended discharge of a chambered cartridge cooking off in a gun barrel heated by firing previous cartridges. Activating the trigger of such firearms releases the spring-loaded bolt to move forward stripping a cartridge from the magazine into the chamber. The firing pin impacts the primer as the cartridge is chambered, and the energy released by the discharging cartridge returns the bolt into an open position while ejecting the empty case.[1]

  1. ^ Johnson, Melvin M. (1944). Rifles and Machine Guns. New York: William Morrow and Company. pp. 352–357.