Firearm malfunction
A firearm malfunction (also misfire and jam) 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. Malfunctions are sometimes considered a factor in negligent discharge. Proper cleaning and maintenance of a firearm play a big role in preventing malfunctions.
Cartridge malfunctions
Case head separation
Case head separation occurs when the walls of the casing become thin or fatigued. Upon firing the round, the case separates into two pieces near the head. It is not uncommon with brass that has been reloaded several times.
Dud
A dud (also a misfire or failure to fire) occurs when the trigger is pulled but the primer or powder in the cartridge malfunctions, causing the firearm not to discharge. Dud rounds can still be dangerous and should be deactivated and disposed of properly.
Hang fire
A hang fire (also delayed discharge) is an unexpected delay between the triggering of a firearm and the ignition of the propellant. Whenever a firearm fails to fire, but has not clearly malfunctioned, a hang fire should be suspected. When this occurs, the correct procedure is to keep the firearm pointed downrange or in a safe direction for thirty to sixty seconds, then remove and safely discard the round (which is now a dud as explained above if the primer was struck, otherwise the gun itself may have malfunctioned). The reason for this is that a round functioning outside of the firearm, or in the firearm with the action open, could cause serious fragmentation hazard.
Squib load
A squib load (also squib round, squib, squib fire, insufficient discharge, incomplete discharge) is an extremely dangerous malfunction that happens when a fired projectile does not carry enough force and becomes stuck in the gun barrel instead of exiting it. In the case of semi-automatic or automatic weapons, this can cause successive rounds to be fired into the projectile obstructing the barrel, which can cause catastrophic failure to the structural integrity of the firearm, and pose a threat to the operator or bystanders. The bullet from a squib stuck in the barrel must never be cleared by subsequently attempting to fire a live or blank round into an obstructed barrel. Furthermore, blank rounds use a type of powder different from that of other rounds, and generate much more pressure. Therefore, the presence of a bullet or other obstruction in the barrel may cause the firearm to fail.
Mechanical malfunctions
Mechanical malfunctions of a firearm (commonly called jams) include failures to feed, extract, or eject a cartridge; failure to fully cycle after firing; and failure of a recoil- or gas-operated firearm to lock back when empty (largely a procedural hazard, as "slide lock" is a visual cue that the firearm is empty). In extreme cases, an overloaded round, blocked barrel, poor design and/or severely weakened breech can result in an explosive failure of the receiver, barrel, or other parts of the firearm.
Failure to feed
Failure to Feed (FTF) is when a firearm fails to feed the next round into the firing chamber. Failure to feed is common when the shooter does not hold the firearm firmly ("limp-wristing").
Hammer follow
Hammer follow occurs when the disconnector allows the hammer to follow the bolt and firing pin into battery, sometimes causing the firing mechanism to function faster than it is designed to. This is usually a result of extreme wear or outright breakage of firing mechanism components, and can result in uncontrollable full-auto operation.
Slamfire
A slamfire is a premature, unintended discharge of a firearm that occurs as a round is being loaded into the chamber.
Stovepipe
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A failure to eject, commonly known as a stovepipe or smokestack, occurs in pump action firearms as well as semi and fully automatic firearms that fire from a closed bolt when an empty cartridge case jams in the ejection port instead of being thrown clear. Stovepipes can be caused by a malfunctioning or defective extractor, or when the shooter does not hold the firearm firmly enough for the action to function fully, known as limp wristing.
Double Feed (Type 3 Malfunction)
The double feed occurs when the spent casing either remains lodged in the chamber or returns to the chamber after firing AND the slide picks up a second round from the magazine mashing it into the first.
Prevention
Some mechanical malfunctions are caused by poor design and cannot easily be avoided. Some malfunctions with cartridges can be attributed to poor quality or damaged (improper storage, moisture) ammunition. Many malfunctions can be prevented by properly practicing firearm cleaning and maintenance on a regular basis.