Accidental discharge 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 discharge s 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 mistakenly 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 gun s , 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 barrel s 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 .