unintentional discharge

An accidental discharge (AD) occurs when there is a mechanical failure of the firearm. This can include things like firearms that do not have mechanisms to render them drop safe falling a sufficient distance, a firing pin stuck forward, a sear failing, or rounds heating sufficiently to spontaneously ignite in the chamber (as may happen in a closed bolt machine gun).

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. The United States Army automatically considers any unintentional discharge to be negligent under the assumption that a trained soldier has control of his firearm at all times, but the United States Air Force only treats discharges as negligent "when a weapon is fired due to either operator error or a lack of attention to basic safety rules." The British military has several definitions of a negligent discharge, such as an "unauthorised and unintentional or inadvertent" initiation of small arms ammunition that is 9mm in calibre or less, "an individual fir[ing] without an order to do so", or "[a] weapon or pyrotechnic discharge, considered by the Conducting Officer or chain of command, to contravene the approved drills or procedures and contrary to the provisions in Queen's Regulations". JSP 830 defines a discharge as accidental if caused by a weaponry defect or negligent if caused by firer error. Even where a discharge is not negligent, "the prevalence of the phrase 'Negligent Discharge or ND' and a general assumption of its meaning" is such that it is used "as a comprehensive and colloquial phrase to describe any unintentional firing".