PS Magazine - TB 43-PS-682
PS, The Preventative Maintenance Monthly
ISSUE 682
SEPTEMBER 2009
32
PS 682
SEP 09
Timing and Headspacing
Protect the Barrel
One of the most vulnerable parts of the M2 is the barrel, particularly its threads.
If the barrel is tossed around or left to roll around in the back of a truck, its threads
are damaged and you can’t screw in the barrel. Never leave a barrel standing on end
where it can take a fall. During travel, tie down a barrel or block it in place. Don’t
store heavy items on the barrel.
Some units give the barrel threads extra protection by screwing on an old canteen
cap or AOAP bottle. For the canteen cap, push out the cap’s center section and
then screw it on the barrel. For the AOAP bottle, cut the bottle in half—an inch
below the threads—and then screw it on. Keep the cap or bottle on the barrel as
much as possible.
Also check the barrel locking grooves
for wear. If they’re too worn, the barrel
will work loose. If you have doubts
about the grooves, try to turn the barrel
with the bolt in the forward position. If
the barrel turns at all, either the grooves
are too worn or the locking spring is
weak.
Never Dry Fire
all your care
for your m2
will be for
nothing if you
don’t headspace
and time it
every time
before firing or
after changing
the barrel.
bad headspace
and timing
cause ruptured
cartridges,
which can lead
to the m2
exploding.
but before
headspacing
and timing,
first do
these
checks…
no dry firing is
allowed with your m2,
especially without the
barrel installed.
if the bolt slams forward,
it ruins the rear threads of
the barrel extension and the
barrel can’t be screwed in.
ease the
bolt forward
instead of
dry firing.
Poke out center section
Screw cap on barrel or…
…screw AOAP bottle on barrel
Gages–
If the headspace and timing
gages are bent, rusted or pitted,
you can’t accurately gage. Get new
gages with NSN 5250-00-535-1217.
Also check that the gages have been
calibrated in the last year.
Timing nut–
If the timing nut can be
moved with one ±nger or it doesn’t
click as you move it, its spring is
weak and it won’t hold timing. Tell
your repairman.
Barrel locking spring–
If the spring
can’t hold the barrel in place, the
barrel can turn during ±ring and
headspace is lost. So test the spring
by getting the correct headspace and
then trying to unscrew the barrel. If
the barrel turns, the spring is weak or
loose.
Barrel
and
barrel
extension
threads—
If the threads are chipped
or burred, it will be dif±cult to screw
in the barrel. What’s worse, you
may think you’ve screwed in the
barrel, but you haven’t–that means
bad
headspace.
Your
repairman
can usually stone chips and burrs
smooth.
Eyeball locking grooves for wear
682.32-33.indd
1-2
7/21/09
6:16:59 PM
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