FM 23-45 Basic Field Manual
6
(2) Screw barrel into barrel extension (by using point of a cartridge or the
combination tool in barrel notches) until the action will just close (recoiling parts will
go fully forward) without being forced.
(3) Then unscrew barrel two notches.
CAUTION:
Care must be exercised to avoid roughening the barrel surface during the
adjustment. Also, the packing must not bind the barrel, as a false adjustment will
result in such a case.
l
. Position belt feed lever stud over Cam groove in bolt and close cover.
m
. Pull trigger.
NOTES. —
1. H
EAD SPACE.
a
. The head space of a military weapon with a cartridge fully seated in the chamber is the distance between
the base of the cartridge and the face of the bolt.
b
. In Browning machine guns, the head space is adjusted by obtaining the proper distance between the
forward part of the bolt and the rear end of the barrel. The head space adjustment must be checked before firing.
2. EFFECT OF HEAD SPACE. ADJUSTMENT.
a
.
General
. — Probably the most important adjustment of the machine gun is the head space adjustment.
Tests show that shot patterns are not adversely affected by the head space when the guns are adjusted as outlined
above. In fact, better uniformity of shot patterns will be obtained when the guns are operated with the above
adjustment which is based on the fundamental design of the weapon. Tests have also proved that guns may be
damaged and in some cases put out of action by using unapproved methods of adjusting the head space. Many
reports show that difficulties with improperly guided belts and with firing mechanisms have been attributed to
undue concern over head space adjustment.
b
.
Insufficient head space
. — When the head space adjustment is too tight, poor functioning will result, as the
breech lock will' not fully enter its recess in the bolt. This condition may damage the barrel extension, bolt, or breech
lock. Extraction trouble may also occur due to improper timing Or locking and unlocking. Further- with a tight head
space adjustment the gun operates sluggishly because of the binding of the moving parts.
c
.
Excessive head space
. — If the head space is too great, a separation Of the cartridge case may occur. Should
there be any weakness in the head of the cartridge case, such as a split case, the possibility of a rupture is increased
by excessive head space.
3. QUICK HEAD SPACE ADJUSTMENT. — After the head space adjustment has been determined by the method
described in paragraph 11k, the notch in which the barrel locking spring is engaged may be marked with a center
punch. Then to make the correct head space adjustment, during the assembly of the gun, screw the barrel all Of the
way into the barrel extension and then unscrew the barrel until the barrel cocking spring is in the marked notch.
■12. D
ETAILED DISASSEMBLING OF BOLT.
a
. Turn the extractor up and remove it to the left (fig. 5).
b
. Great care should be exercised in removing the driving spring rod from the bolt as
the force of the driving spring when released can easily cause the rod to slip away
