When resetting the headspace and timing of a gun that has been fired, use an asbestos mitt to avoid burns.
(3) With the bolt still retracted, unscrew the barrel two notches (clicks). Release the retracting slide handle and allow the bolt to go forward.
NOTE: At this point, check the barrel for rotation. Attempt to turn the barrel in either direction. The barrel should not turn. If the barrel does turn, stop here and check barrel notches and the barrel-locking spring for damage.
(4) Pull the bolt to the rear with the retracting slide handle and hold. This cocks the weapon. Otherwise, the headspace gauge will not fit.
(5) Holding the retracting slide handle, release the bolt, and slowly return the bolt fully forward (Figure 3-20). Do not press the trigger or let the bolt slam forward.
(6) Retract the retracting slide handle and separate the barrel extension from the trunnion block by a 1/16-inch gap (Figure 3-21).
Figure 3-21. Setting the gap.
Figure 3-21. Setting the gap.
(7) Raise the extractor out of the way to clear the top of the T-slot and try both ends of the go/no-go gauge as shown in Figure 3-22.
NOTE: Insert the go end of the gauge between the face of the bolt and the end of the barrel all the way up to the ring.
(8) If the go end of the gauge enters the T-slot freely to the center ring of the gauge, and the no-go end will not enter, headspace is correct. Remove gauge. Headspace setting is now complete. If the go end of the gauge will not enter the T-slot freely, headspace is too tight. Continue as follows.
(9) Retract the bolt so you can see the barrel-locking lug in the center of the 3/8th-inch alignment hole on the right side of the receiver.
(10) Unscrew the barrel one notch (click).
(11) Return the bolt fully forward.
(12) Recheck headspace (step 9).
(13) Repeat steps 10 through 13 until the go gauge fits but the no-go gauge does not fit.
NOTE: You should not have to unscrew the barrel more than five notches (clicks) beyond the first setting of two clicks. If this condition does occur, turn in the machine gun to your unit armorer for inspection.
(14) If the no-go end of the gauge enters the T-slot, headspace is too loose. Adjust it using the same procedures as above, screwing the barrel into the barrel extension rather than out.
(15) Repeat steps 10 through 13, one click at a time, until the no-go gauge does not fit but the go gauge does fit.
c. Timing. Timing is the adjustment of the weapon so that firing takes place when the recoiling parts are between .020 and .116 inch out of battery to prevent contact between the front end of the barrel extension and the trunnion block. Use the following procedures to set timing.
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