SIG SAUER SWISS ARMS SG552 - Manual (Page 26)

5.2.4. 3-round burst control system
The safety lever (6) is set to "3”. The automatic fire pawl
(11) lies in the groove of the safety shaft (7). The segment
(10) is controlled by the safety shaft (7) and moved upward. By pulling the trigger (8), the trigger rod (4) is withdrawn
and simultaneously retained by the automatic fire pawl
(11). The segment (10) is pressed to the rear by the driving
cam (12) in the trigger (8) so that the pawl (13) is allowed
to register.
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Function
By pulling the trigger (8) and after having overcome the
pressure point, the trigger rod (4) releases the hammer (3)
which strikes the firing pin (9). The trigger rod (4) falls
downwards. The recoiling bolt presses the hammer (3) down and
releases the sear (5) via the release bar (2). The sear (5)
catches the hammer (3).
During the bolt advance, the bolt presses down the
release bar (2). The sear (5) is thereby moved to the rear
and releases the hammer (3) to the trigger rod (4). When
the trigger is released, the trigger rod (4) moves to its
upper end position (refer to illu. 63).
Function
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illu. 67) Trigger mechanism during bolt recoil illu. 66) Striking the firing pin illu. 69) Trigger mechanism, immediately prior to firing illu. 68) Home position, 3-round burst control system