By now, you should realize that the .50-caliber BMG is a complex working machine. The care and maintenance given this gun are critical for safe and continued operation. In addition to normal lubrication and cleaning practices, checking and adjusting the headspace and timing of the weapon are mandatory.

HEADSPACE ADJUSTMENT.- Headspace is the distance between the face of the bolt and the base of a seated cartridge case. The distance is correct when the following conditions are met:

1. The recoiling groups are fully forward.

2. There is no independent rearward movement between the bolt, barrel, and barrel extension. Improper headspace adjustment can cause a lot of problems. It causes the gun to operate improperly and, frequently, causes damage to the weapon or injury to personnel.

Headspace must be checked and set before the gun is fired. Other instances when it must be checked include the following:

1. When the gun is assembled

2. When the barrel or any major group or assembly within the receiver is replaced

3. When there is doubt that correct headspace is set

A special tool is used to check and set the headspace distance. It is called the "headspace gauge" and is part of the headspace and timing gauge set (fig. 13-49). The tool should be kept with the gun at all times. For now, we are only interested in the GO-NO-GO headspace

Figure 13-49.-Headspace and timing gauge set.

gauge. The following steps explain how to check and set the headspace adjustment:

1. With the cover group closed, cock the gun. Do so by pulling the retracting slide handle all the way to the rear.

2. Depress the bolt latch release and slowly ease

the restricting slide handle and bolt all the way forward.

3. Raise the cover group. Pull back on the retracting slide handle slightly. Move the bolt not more than 1/1 6 inch to the rear. That prevents the driving spring group and the weight of the parts from giving a false reading. Raise the extractor.

4. Insert the GO end of the headspace gauge into the T-slot. The T-slot is between the face of the bolt and the rear of the barrel, as shown in figure 13-48. The GO end of the gauge should enter the T-slot freely up to the center ring of the gauge. Remove the gauge and try to insert the NO-GO end into the T-slot. If the NO-GO end does not enter the slot, headspace distance is correct.