Slamfire

Activating the trigger of such firearms releases the spring-loaded bolt to move forward stripping a cartridge from the magazine into the chamber.

Some unintentional slamfires are caused by firing pin malfunction, while others may be attributed to defective cartridges with improperly positioned or unexpectedly sensitive primers.

Although the amount of forward movement is not intended to detonate the primer, slamfires occur when conditions do not match design assumptions.

Primers intended to respond to the comparatively light firing pin impact of older or smaller manually-loaded firearms may be initiated by the relatively energetic self-loading process necessary to reliably chamber cartridges.

[5] Spring-loaded firing pins may move further forward than expected during the loading process if the restraining spring is broken or weakened by age.

If a slamfire does occur, the shooter must do his or her best to hold the firearm securely pointed in a safe direction until it ceases firing.

Aside from the dangers of any accidental discharge, an out-of-battery ignition may occur if a round slamfires before it is completely secured in the chamber.

No trigger or lockwork is required, because the loaded barrel is simply inserted into the pipe and slammed backward to fire.

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 it is moving forward before it is fully chambered.
The free-floating firing pin of AR-15 style rifles typically causes an indentation as the firing pin lightly strikes the primer when the bolt closes as it chambers a loaded cartridge. Although a single light indentation may not detonate the primer, [ 2 ] the indentation may make the primer more sensitive to subsequent impact. Some shooters avoid using cartridges with primer indentations from being previously loaded but not fired.