Unintentional discharge

An accidental discharge (AD) may 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 unintentionally left in the chamber. Unintentionally leaving a firearm loaded is more likely to occur when the individual handling the gun is poorly trained.

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."[1][2]

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.

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 STEN 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.

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 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.

A negligent discharge (ND) is a discharge of a firearm involving culpable carelessness. In judicial and military technical terms, a negligent discharge is a chargeable offence. A number of armed forces automatically consider any accidental discharge to be negligent discharge, under the assumption that a trained soldier has control of his firearm at all times. This is the case in the United States Army,[3] Canadian Army, the Royal Air Force, the British Army and various Police Forces within the United Kingdom.

From an article on a U.S. Air Force website:

A negligent discharge occurs when a weapon is fired due to either operator error or a lack of attention to basic safety rules.[4]

• Safety area
• Queen's Regulations and Orders for the Canadian Forces

Forensic firearm examiners typically use more simplistic definitions limited to only 2 categories: unintentional discharge (no mechanical malfunction involved) and accidental discharge (mechanical malfunction involved).

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• Heard, Brian J. (17 August 2011). "4.7 Accidental Discharge". Handbook of Firearms and Ballistics: Examining and Interpreting Forensic Evidence. John Wiley & Sons. ISBN 978-1-119-96477-3.
• Hendrick, Hal W.; Paul Paradis; Richard J. Hornick (26 November 2007). "Chapter 2: Human Factors Causes of Unintentional Shootings". Human Factors Issues in Handgun Safety and Forensics. CRC Press. ISBN 978-1-4200-6297-7.
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• O'Neill, J., Hartman, M. E., O'Neill, D. A., & Lewinski, W. J. (2018). Further analysis of the unintentional discharge of firearms in law enforcement. Applied Ergonomics, 68, 267-272. doi: 10.1016/j.apergo.2017.12.004
• O’Neill, J., Hartman, M. E., O’Neill, D. A., & Lewinski, W. J. (2018, April). The ABCs of unintentional discharges. The Police Chief, 85, 14-15. Retrieved from http://www.policechiefmagazine.org/research-in-brief-the-abcs/
• O’Neill, J. (2018). Functional behavior assessment of the unintentional discharge of firearms in law enforcement. Journal of Organizational Behavior Management, 38(4), 275-287. doi:10.1080/01608061.2018.1514348
• Paulsen, J., (2017, March). 300 Negligent Discharges: Comprehensive Data Science. ConcealedCarry.com. Retrieved from https://www.concealedcarry.com/safety/300-negligent-discharges-comprehensive-data-science-reveals-gun-grabbers-and-gun-owners-are-both-wrong/