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A slamfire is a premature, (usually) unintended discharge of a firearm
A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 that occurs as a round is being loaded into the chamber
Chamber (weaponry)
In firearms, the chamber is that portion of the barrel or firing cylinder in which the cartridge is inserted prior to being fired. Rifles and pistols generally have a single chamber in their barrels, while revolvers have multiple chambers in their cylinders and no chamber in their barrel...

. Slamfires are most common in military firearms that have a free-floating firing pin
Firing pin
A firing pin or striker is part of the firing mechanism used in a firearm or explosive device e.g. an M14 landmine or bomb fuze. Firing pins may take many forms, though the types used in landmines, bombs, grenade fuzes or other single-use devices generally have a sharpened point...

, as opposed to a spring-loaded one. In the action of a typical semi-automatic firearm
Semi-automatic firearm
A semi-automatic, or self-loading firearm is a weapon which performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine...

, the energy of a fired round forces the bolt
Bolt (firearm)
A bolt is a mechanical part of a firearm that blocks the rear of the chamber while the propellant burns.In manually-operated firearms, such as bolt-action, lever-action, and pump-action rifles and shotguns, the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas...

 and bolt carrier rearward, ejecting the empty case. A spring then forces the bolt forward again, and in the process a fresh round is stripped out of the magazine. When the face of the bolt hits the head of the chamber, unless there is a spring around the pin to retard its movement, inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...

 causes the firing pin to continue forward until it is stopped on the primer
Percussion cap
The percussion cap, introduced around 1830, was the crucial invention that enabled muzzleloading firearms to fire reliably in any weather.Before this development, firearms used flintlock ignition systems which produced flint-on-steel sparks to ignite a pan of priming powder and thereby fire the...

 of the round. Sometimes this inertial force is sufficient to set off the primer, thereby firing the round without the operator pulling the trigger. In semi- or fully automatic firearms this can potentially cause the firearm to fire continuously until the magazine
Magazine (firearm)
A magazine is an ammunition storage and feeding device within or attached to a repeating firearm. Magazines may be integral to the firearm or removable . The magazine functions by moving the cartridges stored in the magazine into a position where they may be loaded into the chamber by the action...

 has been emptied or the firearm malfunction
Firearm malfunction
A firearm malfunction is the partial or complete failure of a firearm to operate as intended. Malfunctions range from temporary and relatively safe situations, such as a casing that didn't eject, to potentially dangerous occurrences that may permanently damage the gun and cause injury or death...



Slamfires are extremely dangerous, as the shooter may lose control of the firearm when a normally semi-automatic firearm "goes full auto" unexpectedly. In particular, there is a major risk of a semi-auto gun being uncontrollably propelled backwards by the recoil and firing rounds at the shooter's back as well as any bystanders. Slamfires are very rare in firearms using quality ammunition and not altered outside of manufacturer specs (especially triggers adjusted to an overly light pull weight). Nevertheless, some guns garnered notoriety for tendency to slamfire, with vanilla SKS
The SKS is a Soviet semi-automatic rifle chambered for the 7.62x39mm round, designed in 1943 by Sergei Gavrilovich Simonov. SKS-45 is an acronym for Samozaryadnyj Karabin sistemy Simonova, 1945 Simonov system, 1945), or SKS 45. The Sks is a scaled down version of the PTRS-41 anti-tank rifle also...

 rifles being the best known one. Some shooting ranges require shooters with SKS rifles to load no more than two rounds inside the magazine at any time for just this reason or ban them entirely.

It is always wise to ensure that the firearm is pointed in a safe direction ("downrange") before closing the bolt and chambering a cartridge. If a slamfire does occur, the shooter must do his best to remain calm and hold the firearm securely, pointed in a safe direction, until it ceases firing. Needless to say, this requires extraordinary discipline, and many operators will "spook" and instinctively drop the firearm as soon as it begins firing.

Aside from the dangers of any accidental discharge, slamfires present the particular risk of an out-of-battery
Out-of-battery refers to the status of a weapon before the action has returned to the normal firing position. The term originates from artillery, referring to a gun that fires before it has been pulled back into its firing position in a gun battery....

 detonation. This occurs when a round is fired before it is completely secured in the chamber, and can cause a breech explosion, leading to the firearm being destroyed or damaged, as well as potentially injuring the operator and bystanders.

Possible causes

As dirt and fouling accumulate in the firing pin channel, the pin may begin to protrude from the bolt face, and the risk of slamfire increases. If the firing pin is permanently locked forward the round will fire every time the bolt closes.

Pump action shotguns lacking a trigger disconnector
Trigger disconnector
A trigger disconnector is a type of safety in a firearm, which prevents the trigger from being held down after a shot has been fired. This disallows either automatic fire or deliberate "slam fire", depending upon the particular firearm's action. Notable firearms that lack this feature include...

 can be slamfired, on purpose or otherwise, if the trigger is kept depressed whilst cycling the action.


Recognizing the obvious need to minimize the chance of this happening, there are two simple methods commonly employed. One is to use ammunition with harder primers, which require a more significant strike from the firing pin and are thus unlikely to go off with a comparatively light inertial strike. Most military ammunition makes use of hard primers for this reason. The second method of minimizing the risk of slamfire is in the design of the firearm itself. Spring-loaded firing pins prevent slamfires because the pin cannot easily move inside the bolt. A proper strike from the firearm's hammer will overcome the resistance of the spring, but ordinary inertia will not. A simple solution in free-floating pin designs is to make the firing pin itself very lightweight, which reduces its inertia and lessens the risk of slamfire.

In properly maintained firearms using appropriate ammunition, slamfires are very rare.

Firearm design issues

Technically, the risk of slamfire is inherent in any design that uses a free-floating firing pin. This does not necessarily equate to an inferior design, however, as many excellent military firearms make use of them due to their simplicity and ruggedness. For example, the Winchester Model 1897
Winchester Model 1897
The Winchester Model 1897, also known as the Model 97, M97, or Trench Gun, was a pump-action shotgunwith an external hammer and tube magazine manufactured by the Winchester Repeating Arms Company. The Model 1897 was an evolution of the Winchester Model 1893 designed by John Browning. From 1897...

 shotgun was deliberately designed to be slamfired by holding the trigger and operating the pump. This permitted a soldier to rapidly spray shells onto a target area (such as when attacking a trench). Ithaca 37
Ithaca 37
The Ithaca 37 is a pump-action shotgun made in large numbers for the civilian, military, and police markets. It utilizes a novel combination ejection/loading port on the bottom of the gun which leaves the sides closed to the elements. In addition, the outline of the gun is clean...

 shotguns were used by US Navy SEAL
Seal commonly refers to:* Pinniped, a diverse group of semi-aquatic marine mammals many of which are commonly called seals* Seal , a device which helps prevent leakage, contain pressure, or exclude contamination where two systems join...

 units during the Vietnam war due to this capability.

Another design often associated with a tendency to slamfire is the SKS
The SKS is a Soviet semi-automatic rifle chambered for the 7.62x39mm round, designed in 1943 by Sergei Gavrilovich Simonov. SKS-45 is an acronym for Samozaryadnyj Karabin sistemy Simonova, 1945 Simonov system, 1945), or SKS 45. The Sks is a scaled down version of the PTRS-41 anti-tank rifle also...

. Many of the reported cases of slamfire in the SKS are quite likely in surplus examples that have not been properly cleaned of preservative cosmoline
Cosmoline is the genericized trademark for a generic class of rust preventatives, typically conforming to MIL-C-11796C Class 3, that are a brown colored wax-like mass; have a slight fluorescence; and have a petroleum-like odor and taste .Chemically, cosmoline is a homogeneous mixture of oily and...

 inside the firing pin channel. While the fix is relatively simple and inexpensive (a modified firing pin with a spring to counter forward inertia), it's impossible to tell from the outside if it has been applied.

In 2006 FN Herstal recalled the FS2000 semi-automatic carbine that had been inadvertently assembled with the firing pin of the FN F2000 military carbine. The military firing pin was a heavier design, used to achieve complete ignition reliability with the harder or denser primers generally used in military-specification ammunition. The use of this military firing pin reportedly caused slam-fires in the FN FS2000 when firing with certain types of commercial ammunition available in the United States.