Accidental discharge is the event of a firearm discharging (firing) at a time not intended by the user. Perhaps most commonly, accidental discharges (sometimes called ADs by military and police personnel and referred to as negligent discharges by several armies) occur when the trigger of the firearm is deliberately pulled for a purpose other than shooting—dry-fire practice, demonstration, or function testing—but ammunition is negligently left in the chamber. Unintentionally leaving a firearm loaded is more likely to occur when the individual handling the gun is poorly trained, and perhaps also with removable-magazine-fed firearms (as the magazine may be removed, giving an unloaded appearance even when a round remains chambered—see discussion of magazine-safeties below).
A second common cause of accidental discharges is when the gun-handler places his finger on the trigger before he has decided to shoot. With the finger so positioned, many activities may cause the finger to compress the trigger unintentionally. For example, if one attempts to holster the firearm with finger on trigger, the holster edge will drive the finger onto the trigger, and discharge is likely. If one stumbles or struggles (with an adversary) with finger on trigger, the grasping motion of both hands will likely cause the trigger finger to compress the trigger.
Gun safety rules recognize the above possibilities, and aim to prevent them. The primary firearm safety rule usually listed by any source is a version of "Always keep the gun pointed in safe direction." Following this rule ensures that, should an accidental discharge occur, no harm will be done. Second, one's finger should remain outside the trigger guard until the decision has been made to fire the weapon; even if (some would say especially if) one is facing a hostile adversary and has to be prepared to fire at a moment's notice, keeping the finger out of the trigger-guard until one has decided to shoot will prevent an accidental shooting.
When a firearm is not in use, storing it unloaded and in a separate container from ammunition may also help prevent accidental discharges, especially if the stored firearm is "accessed" by an unauthorized user.
On occasion, an accidental discharge can occur by means other than the finger pulling the trigger, such as dropping a loaded weapon. Because of this possibility, most currently produced pistols are designed with a "drop-safety" or firing pin block, a mechanism inhibiting or isolating the firing pin, preventing accidental discharge if the firearm is dropped. However, most long guns do not have drop- safeties.
Prevention of dropped-firearm ADs with long guns therefore depends on the user being familiar with the precautions needed for that particular gun: it is standard practice for all long-gun users to unload the firearm's chamber before any activity that might foreseeably result in a dropped firearm (e.g., climbing a fence while hunting), and before placing the firearm in a vehicle (where sudden deceleration may cause the firearm to act as if dropped).
Accidental discharges not involving trigger-pull can also occur if the firearm is mechanically unsound: poor maintenance, abuse, inept "gunsmithing," or the use of substandard materials or defective ammunition in the gun may all lead to breakage.
In World War II, early versions of the British Sten gun were notorious for accidentally discharging when dropped. While dropping any long gun with loaded chamber can be dangerous, the use of a soft bronze bolt in early Stens—allowing the sear contact area to wear down quickly—made ADs even more likely.
As the early Sten gun illustrates, "poor design," including the specification of unsuitable component materials, is sometimes cited as a cause for some firearms being "unsafe." Surely, any firearm that can discharge by itself when loaded is unsafe, and no user would want to handle a firearm that has a high likelihood of accidental discharge when employed for its intended use. But, firearm designs incorporating fewer safeties are not intrinsically unsafe—see discussion of long-guns above—even if they do require competence and proper training for safe use.
A relevant discussion attends the use of magazine-safeties in semi-automatic pistols. Such safeties prevent the pistol from firing if there is a round in the chamber, but the magazine is removed. Some self-defense experts dislike this feature, as it may allow the pistol to be "disabled" unexpectedly during a struggle, so that the pistol fails when needed. Others feel that it may improve user safety and prevent some accidental discharges. Both semi-auto pistol designs, with and without magazine-safeties, are currently manufactured.
One last form of accidental discharge, known as cooking off, occurs when a weapon becomes overheated, with the firing chamber hot enough to ignite the propellant charge in the ammunition round, causing the cartridge to fire. As recently as 1979, this problem doomed the innovative Heckler & Koch G11 during NATO trials.
"Cooking off" is typically encountered only in fully automatic weapons, such as machine guns, when they are fired for long periods of time without allowing the barrel and chamber of the weapon to cool down to safe temperatures. For this reason, modern crew-served machine guns are equipped with spare barrels to allow a machine gun crew to replace an overheated barrel with a cool one, thus restoring the weapon to action while the overheated barrel is allowed to cool. Also, the majority of machine guns fire from an open bolt, so the round is chambered only after pressing the trigger, just before firing.
- Video: Accidental discharge during firearms class
- NRA Gun Safety Rules
- Ayoob, M: "The Subtleties of Safe Firearms Handling"
(broken Youtube link)