Before examining any firearm, the examiner must thoroughly understand the cycle of fire. This refers to the steps involved in discharging a firearm. Regardless of action type or design, the operation of all firearms follows the steps outlined in the cycle of fire.
The steps in the cycle of fire are as follows:
The elements of these steps can be traced in the firing of any breechloading weapon. In lower velocity firearms, some of the steps may be simplified or combined, but are nevertheless present in principle.
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 .3006 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 pressureformfitted to the chamber).
Likewise, the primer cup has been pressureheld against the side of the cartridge case and depends upon the face of the breechblock for locked support during the interval of highchamber 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.
Step 6: Unlocking
This is the reverse of the locking process and is frequently performed in conjunction with extraction.
Step 7: Extraction
Although cartridge cases do not commonly exceed their elastic limit during firing, they have a tendency to stick to the chamber after firing. After firing, cartridge cases are larger in diameter than before firing. If the fired cartridge case is intended for reloading, it must be full-length resized in a reloading die.
The removal of a case from the chamber must begin with a slow movement that can be accelerated as soon as the case loosens from the chamber. This slow-to-fast action can be demonstrated by carefully observing the extraction process in a highvelocity bolt-action rifle.
All cartridge cases are designed with a rim or groove (cannelure) at the base so that an extractor claw can grasp this edge in order to achieve extraction.
Step 8: Ejection
In the final stages of extraction, the cartridge case encounters a projection that is usually at right angles to the exit portal of the breech. Rotating on the fulcrum of the extractor, the case base is contacted on the opposite side by the ejector, which flips the case out of the actuating mechanism.
Step 9: Cocking
The hammer spring is usually cocked when the bolt of a rifle, pistol, or repeater shotgun is retracted. An exception to this is the M1917 Enfield Rifle, which cocks upon forward motion of the bolt. Exposed hammers may be cocked by manual retraction, using the thumb. The Walther series of pistols provides for manual cocking or trigger pull cocking (double action), as do most open hammer revolvers.
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