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A slamfire is a premature, (usually) unintended discharge of a firearm that occurs as a round is being loaded into the chamber. Slamfires are most common in military firearms that have a free-floating firing pin, as opposed to a spring-loaded one. In the action of a typical semi-automatic firearm, the energy of a fired round forces the bolt 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 causes the firing pin to continue forward until it is stopped on the primer 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 has been emptied or the firearm malfunctions.
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 SKS 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 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 and solutions
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 stuck forward the round will fire every time the bolt closes.
Also, any rifle with a free-floating firing pin will have the firing pin lightly strike the primer when the bolt closes as it chambers in a fresh round. This is perfectly normal, but trouble arises when the gun is loaded with ammo containing primers that are much more sensitive than the ones in the ammo the gun was designed for.
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 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 shotguns were used by US Navy SEAL units during the Vietnam war due to this capability.
Another design often associated with a tendency to slamfire is the SKS. 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 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.
Slam fire in paintball
In the game of paintball, slam fire is when the paintball marker's trigger is held in and for every manual pump cycle a ball will be discharged. Significant degradation in aim precision borne from the vigorous pumping is compensated by elevated rate of fire, useful at close range and not available from a pump action paintball marker in any other way.
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