Figure 13-48.--The .50-caliber BMG receiver; cutaway view.
You can follow most of the operating cycle of the
Headspace and Timing Adjustments
gun by referring to figure 13-48. Assume the chamber
By now, you should realize that the .50-caliber
is loaded, the gun is cocked, and the bolt latch is
BMG is a complex working machine. The care and
released. When the trigger is depressed, the trigger bar
maintenance given this gun are critical for safe and
pivots and releases the cocked firing mechanism. The
continued operation. In addition to normal lubrication
spring-loaded firing pin strikes the primer and the
and cleaning practices, checking and adjusting the
cartridge fires. Pressure from the expanding gases
headspace and timing of the weapon are mandatory.
causes the recoiling parts of the gun to start moving
HEADSPACE ADJUSTMENT.-- Headspace is the
distance between the face of the bolt and the base of a
During the first 3/4 inch of rearward travel, the
seated cartridge case. The distance is correct when the
recoiling parts remain locked together. However, the
following conditions are met:
breech lock depressors are acting on the breech lock pin.
That action forces the breech lock down and out of the
1. The recoiling groups are fully forward.
bolt. As a result, at the end of the first 3/4 inch of recoil,
2. There is no independent rearward movement
the bolt is unlocked. It is free to continue recoiling
between the bolt, barrel, and barrel extension.
independent of the barrel and barrel extension.
Improper headspace adjustment can cause a lot of
The barrel extension hits the accelerator. It, in turn,
problems. It causes the gun to operate improperly and,
hits the bolt and accelerates it (the bolt) to the rear. The
frequently, causes damage to the weapon or injury to
barrel and barrel extension recoil another 3/8 inch
(1 l/8-inch-total travel). They are stopped by the buffer
Headspace must be checked and set before the gun
is fired. Other instances when it must be checked include
Meanwhile, the bolt recoils an additional 6 3/8
inches to the rear (7 1/8-inch-total travel). During this
movement, the driving spring group is compressed and
1. When the gun is assembled
the bolt is stopped by the bolt buffer mechanism. The
2. When the barrel or any major group or assembly
fired cartridge is extracted and the firing mechanism is
within the receiver is replaced
3. When there is doubt that correct headspace is set
Counter recoil forces the-bolt forward and the empty
A special tool is used to check and set the headspace
cartridge case is ejected. The bolt locks to the barrel
extension and both move forward into the battery.
distance. It is called the "headspace gauge" and is part
Feeding and cambering have taken place and the gun
of the headspace and timing gauge set (fig. 13-49). The
is ready to fire. The cycle begins when the firing pin is
tool should be kept with the gun at all times. For now,
released to set off the next cartridge.
we are only interested in the GO-NO-GO headspace