Steps 1-5

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Step 1: Feeding

Feeding refers to the insertion of cartridges into the chamber; the breech bolt pushes the cartridge into final position. Typically, the incoming round slides across the bolt or breech face during this camming action. The feeding function can be manual or performed by various kinds of magazines and clips. For example, machine guns use belts of cartridges.

Step 2: Chambering

Chambering is the insertion of the cartridge into the chamber. If a cartridge of the incorrect length or diameter is used or if there is foreign matter in the chamber, chambering may be obstructed, causing a malfunction. Excess oil or grease in the chamber may cause overpressure, resulting in a ruptured cartridge case and potentially serious accidents.

Step 3: Locking

The breech bolt mechanism locks the cartridge into position in the barrel before firing. Most quality firearms are equipped with an interrupter mechanism that disconnects the trigger from the firing pin, thus making it impossible to fire until the mechanism is safely locked. This critical relationship is referred to as timing. (Blowback mechanisms involve a spring-held bolt; the mechanism is not technically locked, it is held together by spring tension and bolt inertia.)

The locking principle is easily demonstrated by closing a high-velocity .30 caliber bolt-action rifle. When the bolt is turned down at the end of its forward thrust, one or more lugs rotate into machined slots or against a shoulder in the receiver. This closure is essential; if the firing pin falls on the cartridge primer before the mechanism is safely locked, an accident may occur.

Step 4: Firing

When the breech is fully locked, a pull on the trigger mechanically translates to the firing pin release.

In the cocked position, the firing pin has a hammer behind it with a spring forcing it towards the primer, restrained only by a sear that is engaged by the trigger. A pull on the trigger trips the sear from the engaging notch in the hammer. The hammer, actuated by a cocked spring, drives the firing pin sharply against the percussion‑sensitive primer, which fires the cartridge.

Step 5: Obturation

Obturation occurs when powder gases under high pressure (e.g., two and one-half tons per square inch in the .30‑06 Springfield cartridge) are sealed to prevent them from jetting between primer cup and cartridge case, cartridge case and primer wall, and projectile and bore.

Cartridge cases must be sufficiently flexible to expand against the chamber wall and transmit the instantaneous powder pressure to the barrel metal that surrounds the chamber. When the chamber pressure has returned to zero, the cartridge case must also be flexible enough to release itself from the chamber wall (even though it is now pressure‑form‑fitted to the chamber).

Likewise, the primer cup has been pressure‑held against the side of the cartridge case and depends upon the face of the breechblock for locked support during the interval of high‑chamber pressure.

Obturation also occurs with the projectile; bullets are made sufficiently larger than the bore diameter to extrude into the rifling grooves and seal the gases. The sharp hammer action of the instantaneous high pressure and temperature may upset the projectile base, which enhances sealing. Shotgun wads perform the sealing function in smooth bore weapons.

Click here to watch a video on steps 1-5 of the cycle of fire.

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