Slamfire

A slamfire is a premature, 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 weapon, 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. This can potentially occur repeatedly, until the magazine has been emptied. Slamfires can create a very dangerous situation, in which the shooter loses control of a firearm that is discharging automatically. 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.

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 damage or destroy the firearm, as well as potentially injure the operator and bystanders.

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.

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. One such 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.

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