SIG SAUER SWISS ARMS SG552 - Manual (Page 24)

The bolt assembly moves back along the rails in the
receiver (16) as far as the stop (17) whereby:
• the recoil spring (2) is compressed;
• the hammer (7) is cocked;
• the extractor (18) extracts the case from the chamber;
• the ejector (19) ejects the case through the port in the
receiver (16).
5.1.4. Bolt advance
The force of the compressed recoil spring (2) thrusts the
bolt forward. The bolt head (5) feeds the next round from
the magazine (20) into the chamber.
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Function
5.1.3. Unlocking and recoil of bolt
The gas pressure, generated by the burning powder, drives
the bullet through the barrel (13). As soon as the projectile
passes the gas port (15), propellant gas flows through the
adjustable gas valve (14). The gas pressure acts on the
gas piston (1) which pushes the bolt carrier (4) to the rear. During the rearward motion of the bolt carrier (4) the bolt
head (5) is rotated by the control cam (3) so that the locking
lugs (8) are disengaged. The bolt is now unlocked.
Function
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illu. 59) Unlocking begins illu. 60) Case ejection illu. 61) Bolt advance illu. 58) Bolt carrier recoil begins