The original AK operating system
Thought Kalashnikov designed several assault rifles the Russian army never dropped the weapon system from the moment is was officially adopted 1947. The operating systems is one of the aspects that makes the Kalashnikov assault rifles so reliable. There are a few reasons for that. From a field disassembly perspective there are no small parts incorporated in the system and so no small significant parts could get lost, damaged or break under the tremendous cycle forces. Practically all assault rifles designed by Mikhail Kalashnikov share the same operating system.
The system comprises four main parts namely: the bolt, the bolt carrier, the gas piston rod and the return spring. The bolt carrier is connected with the operating rod which in his turn is permanently attached to the piston. So in fact these three parts are integrated into a single part. All these parts are relatively heavy by which these parts are still able to function even if dirt like grit or mud gets into the system. The system is a very simple thought-out design robust which is hardly prone to malfunctions it is easy to clean and maintain.
Just because the system is so simple it makes it rather easy to explain and teach soldiers how to use and maintain their AK. Another aspect is that the system has wide tolerances. Though this would affect the accuracy of the weapon the AK is accurate enough for the modern soldier. Stories that the AK are far less accurate than Western design are false. The AK can compete with current assault rifles. In short the system works as follows: when the trigger is pulled the hammer strikes the primer of the cartridge. The powder burns which creates propellant gases. The propellant gases force the bullet to pass through the barrel. When the bullet passes the gas vent a fraction of the propellant gases escape into gas port bounching against the piston head by which the bolt carrier is forced backward unlocking the bolt out the locking recesses. The bolt is also travelling with the bolt carrier in a backward direction taking the empty case out of the chamber. The spent case is ejected after it bounces on the fixed ejector. (look at the upper Youtube animation movie to see how the process is working, see it in slow motion as from 00:22).
Derivative AK operating systems
The Kalashnikov system has been used in several other small arm types. The system is sometimes partly redesigned but still a clear relationship with the Kalashnikov system is there. In the Kalashnikov Encyclopaedia all weapons included are categorized using the Kalashnikov classification flowchart. With this methology the intensity of the technical relationship with one of the original Russian AKs is determined. One of the systems that has been included is the SIG SG550 series. No parts are interchangeable between this weapon series and the Russian AK, however conceptually the Swiss operating system has much in common with the AK.
One of the most related weapons from the SIG SG550 series is the SG552. The operatng system of this variant is even more recognizable as being based on the Kalashnikov system. The following picture clearly indicates this. In the middle of the picture the Kalashnikov system parts are shown. There above the operating system of the SG550 is presented and beneath the Kalashnikov system the SG552 operating parts. The picture clearly reveals that the SG552 has more close family ties because the return spring of the system is located behind bolt carrier.
Two interpretations of the Kalashnikov system, in the middle the original AK system above that the AK based system of the SG550 the lower is the system of the SG552 notice the similarities between the three systems.