Firearm Function Testing

It is not too uncommon in shooting investigations for a suspect or victim to claim that a firearm accidentally discharged when it was dropped or otherwise mishandled. The accidental discharge of a firearm can occur and most of the time the only harm done is a hole shot in Uncle Pete's pick-up truck. However, all too frequently deaths and serious injuries result from the accidental discharge of a firearm.Harley-Davidson Leather Vest

This vest contained a derringer in its right front pocket. The wearer removed the vest and tossed it to a table. The derringer's hammer struck the table and the derringer discharged causing the large hole seen in the picture below. The bullet struck the owner in the chest, killing her instantly.

Hole in vest.The firearm examiner's job usually involves the inspection of submitted firearms to determine if there is any basis for a claim of accidental discharge.

When a firearm is submitted to the laboratory there are a number of tests performed to determine if the firearm is functioning properly. The firearm is inspected for damaged, worn, or missing parts and it is examined to determine what safeties, if any, are incorporated into the design of the firearm.Derringer Safety

In the case mentioned above, the derringer was not found to have any mechanical deficiencies. However, the derringer had a safety that required the operator to manually cock its hammer back into a half-cocked position prior to loading. Without engaging the safety the hammer would be resting directly against the firing pin of one of the derringer's chambers.

Most modern manufactured and a great number of antique firearms have some type of safety designed to minimize the potential for accidental discharge.  There are some firearms however, that do not have safeties.

Safeties usually act to block the trigger, hammer, or firing pin of a firearm and some even have multiple safeties.  Safeties can come in many forms and can be of the external manually operated variety or can be internal and operate automatically as a function of the firearms action.

External Safeties

One of the most common external safeties is the Cross Bolt Safety as seen below.  This safety is operated by lateral force on a button usually located on the trigger guard of a firearm.  When engaged this safety prevents the trigger of the firearm from being pulled.  This safety is very common in rifles and shotguns.

Another external and manually operated safety, common to semiautomatic pistols, is a Sliding Button Safety.  This safety is operated by a sliding motion and is typically located on the left side of the firearm's frame as seen below.  When engaged this safety can block the trigger and can disengage or block the internal sear.  

Another external safety seen in the image above is called a Grip Safety.  Located on the back of the grip, this safety button is depressed by your hand while gripping the firearm.  If not depressed the grip safety blocks the trigger from being pulled.

The Thumb Safety seen below is another common pistol safety.  This safety usually located on the slide can block the hammer from striking the firing pin, will sometimes de-cock the pistol's hammer, and can disengage the trigger mechanism.

Internal Safeties

The Transfer Bar Safety seen below is found in RUGER and other revolvers.  Connected to the trigger the transfer bar must be in the up position for energy to be transferred from the hammer to the frame-mounted firing pin.  As long as pressure is maintained on the revolver's trigger the transfer bar will be in the up position.  If pressure is not maintained on the trigger the transfer bar will move down and out of the way as the hammer falls and as a result the hammer will not transfer its energy to the firing pin and the revolver will not fire.  This safety will prevent the accidental discharge of the revolver if the cocked hammer were to "jar-off" when dropped. It will also prevent discharge if a blow is delivered to the back of an un-cocked hammer.

The Hammer Block Safety, common to revolvers with hammer-mounted firing pins, is connected to the trigger mechanism and works in a fashion opposite that of the transfer bar safety.  The image below shows a Smith & Wesson revolver with the hammer in the uncocked position.  The hammer block is highlighted in red.  In this position the hammer block is in the up position and will prevent the hammer-mounted firing pin from moving through the frame.

The image below shows the hammer in the cocked position.  The hammer block moves down when the hammer is cocked and as long as the trigger is held to the rear will stay down and out of the way as the hammer falls.  If pressure is not maintained on the trigger it will recover forward as the hammer falls and the hammer block will move up preventing discharge of the revolver.

The image below shows a cut-away of a Glock semiautomatic pistol.  Glock pistols and many other semiautomatic pistols have an internal Firing Pin Safety.

It is a little difficult to see but when the trigger is forward a small internal plunger is blocking the pistol's firing pin.  The plunger must be pushed up to unlock the firing pin.  When the trigger is pulled to the rear the trigger bar pushes the plunger up just before the release of the hammer or firing pin.

Also seen in the image above is the Glock Trigger Safety.  This external safety is a small extension on the forward edge of the trigger must be depressed before the trigger can be pulled.

The safeties described above are only some of the more common varieties. Safeties can be very effective in preventing the accidental discharge of a firearm however, it should be noted that even the most sophisticated firearms have been known to discharge unexpectedly.  Have you ever heard of Murphy's Law?  User error, dirt, damage and/or poor design can contribute to any inadvertent discharge.  Firearm examiners will inspect the firearms collected from shooting incidents to document the overall condition and operability of the firearm in question.


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