Firearms have come a long way from the days of muskets when it would literally take minutes to fire very few projectiles. Nowadays there are weapon systems capable of launching millions of rounds at a target in less than a minute. The biggest step taken in order to reduce weapon's rates of fire, was to remove the human element from its sequence of operations. That is, to make the weapon 'responsible' for its own chambering and cocking.

In this write up, I am basically trying to explain what is happening inside a weapon that allows the operator to simply hold in the trigger and have a constant stream of ammunition come from the barrel. Aside from a few small discrepancies, this description should hold true for the majority of single barrel, gas operated, semi/fully automatic rifles. Such as the M-4 and the AK-47.

Sequence of Operations

1. Firing-

With the weapon's safety catch set to 'fire', when the trigger is squeezed, the hammer in the trigger mechanism is released, which strikes the firing pin. The firing pin then moves through the bolt of the weapon and strikes the primer cap on the base of the round. This ignites the propellant inside the round. The ignition of the powder creates an explosive expansion of gases inside the chamber, which in turn gives the projectile the kinetic energy required to move from the chamber and through the barrel at speeds in excess of 900m/s (3240km/h).

Inside of the barrel of these weapons is a small hole which acts as a vent for the gases which build up behind the projectile. This vent opens up into a small chamber, which contains a piston and spring. Upon entering the chamber, the force from the pressure of the gas is imparted upon the piston, pushing it rearward (opposing the direction of travel of the projectile). The piston then pushes back the connected guide rods which are attached to the bolt. The piston spring then forces its way back to a neutral position.

2. Unlock-

As the bolt is moved rearwards, it is effectively unlocked from the chamber and barrel and can rotate freely. This operation is known as the unlock.

3. Extraction-

The claw on the bolt face is attached to the base of the round. As the bolt is forced rearwards, the claw drags the cartridge back with the bolt, 'extracting' it from the chamber.

4. Ejection-

When the round is fully free of the chamber, it is then pulled further rearwards to the ejection breach. The ejection breach is a rectangular hole on the side of the weapon, opposite the operator's body. As the casing moves parallel to the ejection breach, it is forced out by the ejector pin, which is on the side of the bolt (usually opposite the extractor claw). The spent casing then falls freely to the ground.

5. Cocking-

After the casing has been ejected, the bolt continues rearwards, pushing the firing pin back with it. The firing pin then forces the hammer back into its original position, thereby resetting it. The trigger mechanism has now been cocked.

6. Feeding-

At this point the springs inside the weapon reverse the direction of movement of the bolt and it begins its travel back to its original position. As the bolt moves over the magazine, the catch underneath the bolt collects the round at the top of the magazine and guides it forwards to the chamber. As the round is moved clear of the magazine, the next round is forced upwards to take the position of the previous round which is now on its way to the chamber.

7. Chambering-

As the round is guided forwards, the ejector pin is depressed and the extraction claw is clamped to the base of the round. The round then reaches the chamber.

8. Locking-

The bolt and the round are now fixed in position and the process is ready to be repeated and will continue to do so until the trigger is released.

If the weapon is well maintained and the conditions for use of it are good, this process will be repeated over twelve times in under a second.