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Firing is the sequence of events which ignites the propellant charge in the cartridge. As described in the Definitions section the cartridge primer contains a small amount of percussion-sensitive explosive. When a striker hits the primer with sufficient mechanical shock it detonates the primer material. This sends hot burning particles of primer material into contact with the propellant, beginning its burning. In single-shot and semiautomatic operation, each activation of the trigger permits only one shot. In fully-automatic operation the entire operating cycle repeats while the trigger is pressed and ammunition is fed.

Strikers There are many ways to organize the mechanical firing chain. Probably the most common striker is a slender steel rod called a firing pin. In most cases it is struck at the rear by a rotary hammer. Energized by the hammer, the firing pin then strikes the primer. In other designs, the firing pin is propelled directly by its own spring, or the hammer impacts the primer directly without an intervening firing pin. In modern revolvers the hammer usually hits a transfer bar which is moved into position only when the trigger is pressed. With the transfer bar in proper position, the striker can hit the transfer bar which hits the primer. With the transfer bar out of position, the gun should not discharge inadvertently if the gun is dropped on its hammer or muzzle.

In guns with firing pins, they are sometimes made floating, meaning the firing pin can move freely within the bolt or receiver. A floating firing pin, if massive enough, may cause unintentional ignition if it rides in a forward-moving bolt. The bolt is stopped by the round and chamber, but the firing pin continues to move forward due to its own inertia. If the firing pin's momentum is great enough, the primer can be detonated. To lessen this possibility firing pins are often equipped with springs which force them away from the primer. These springs are weak enough that they do not significantly impede the impact of the hammer on the firing pin. They are strong enough only to counter the inertia of the firing pin itself.

Fixed Striker Perhaps the simplest firing method is a fixed striker in the form of a protuberance on the face of the bolt. The fixed firing pin strikes the primer when the round is chambered. While mechanically very simple, an exposed, fixed firing pin opens the dangerous possibility of striking the primer when the round is not in the chamber. The pin itself can also be damaged since it is exposed.

In an unusual method called advanced primer ignition the fixed firing pin deliberately strikes the primer before the round is fully chambered. While this would seem to violate the principle of allowing combustion only when the breech is fully closed, the reason for it is clever. Since the bolt at that time is still moving forward the round is chambered shortly after ignition. Recall that all of the propellant powder does not ignite instantaneously, rather it burns in an orderly fashion in order to keep chamber pressures within safe limits. With advanced primer ignition the propellant gets a "head start" which more makes efficient use of cycling time and helps shorten it.

Some rounds are electrically ignited. For example, cannon rounds usually have primers with concentric electrical terminals. When a round is chambered, electrical contacts on the breech are pressed into contact with the primer terminals. The rounds are are ignited by passing current through the primer charge.

A different form of electrical ignition used in some competition .22 rimfire arms is to pass current through an electrode or heating element which is in direct contact with the rim. The resulting heat sets off the primer material in the rim. The lock time, that is delay between pressing the trigger and ignition of the round, is said to be greatly shortened. Since a gun held by a human competitor is always moving, however slightly, shortened lock times can increase shooting precision.

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