A slamfire is a premature, usually unintended discharge of a firearm that occurs as a cartridge 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.
Unintended 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 significant risk of a semi-auto gun being uncontrollably propelled backwards by the recoil and firing rounds at the shooter and any bystanders. Slamfires are very rare in properly maintained firearms using quality ammunition and not altered outside of manufacturer specifications (especially triggers adjusted to an overly light pull weight).
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 their 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 become surprised 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 damaged or destroyed, 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.
To avoid the risk of spring-loaded firing pins becoming stuck forward and causing slam fires, it is important to avoid introducing excess lubricant into the firing pin assembly of such weapons, as wet lubricant can trap carbon and other pollutants from burning propellant, causing firing pins to "stick". In fact, the firing pin assemblies of most semiautomatic firearms are designed to require no external lubrication by the user.
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 ammunition containing primers that are much more sensitive than the ones in the ammunition the gun was designed for.
Recognizing the obvious need to minimize the chance of slamfires, 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.
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.
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 cosmoline preservative inside the firing pin channel. While installing a modified firing pin with a spring to counter forward inertia will prevent slamfires, it is impossible to tell from the outside of the rifle if this has been done.
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.
The first civilian semi-automatic version of the Tavor exported to Canada had the same issue. Initial owners were shipped a retaining spring and current versions include this spring.
Intentional slamfire designs
There are intentional slamfire designs, mainly shotguns, that involve a free floating barrel, with a round inserted in the breech, and a tubular receiver with a fixed firing pin in the back. The barrel is inserted into the receiver and slammed backward, firing the round. There is no triger or lockwork.
Many early pump-action shotgun designs such as the Winchester Model 1897, Model 1912 and Itchaca Model 37 lacked a trigger disconnector. Combined with a second sear that allowed the hammer to strike as soon as the action was operated if the trigger was still depressed, this allowed for such a shotgun to fire as fast as the user could pump the action.
- "Collecting and Shooting the Military Surplus Rifle (2006) - Surplusrifle.com". Surplusrifle.com. Retrieved 2008-09-06.
- "CCI Military Rifle Primers". CCI. Retrieved 2009-03-04.[dead link]
- "Archived copy". Archived from the original on 2016-09-16. Retrieved 2016-09-26.