Ordnance Notes -- by Bob Stoner GMCM(SW) Ret.
XM-21 7.62mm NATO Sniper Rifle with Sionics Suppressor
Photo: US Army
XM-21/M-21 Sniper's rifle
as used in Viet Nam. The photo shows the Leatherwood 3X to 9X
Adjustable Ranging Telescope (ART) sight. The Leatherwood ART has a ballistic
cam that is
calibrated for the trajectory of the 7.62mm M-118 Special Ball cartridge. The
M-118 uses a full
metal jacket, 173-grain, .308 inch diameter boattailed bullet that leaves the
barrel of the M-21 at
2,550 feet per second. When fitted with the Sionics suppressor, the front sight/flash
of the rifle is replaced. The Sionics suppressor uses a pressure relief valve
to permit the semi
-automatic action of the M-21 to function without any modifications to the gas
(More photos at bottom of page.).
The XM-21 or M-21 7.62mm NATO sniper's
rifle is an accurized M-14 rifle. The M-14 rifle is a product-improved version
of the venerable M-1 Garand rifle of World War 2 and Korea. The major improvements
of the M-14 over the M-1 were a 20-round magazine vs. an 8-shot clip; no more
"last round" twang as the clip ejected; a charger or stripper clip
guide which allows the magazine to be "topped-off" from 5-round clips
while still in the rifle; a selector switch that allows semi-automatic or full-automatic
fire; a redesigned gas system; a gas cutoff for launching rifle grenades; an
efficient flash suppressor.
The M-21 7.62mm Rifle (Sniper's)
is derived from the M-14 7.62mm Rifle (National Match). The M-14 NM has its
selector shaft welded to permanently convert it to semi-automatic fire only.
The action of the M-14 NM is bedded
and its barrel is free-floated so that none of the operating parts contact the
stock at any point. The trigger pull is adjusted to 4-1/2 pounds. The flash
suppressor is reamed to prevent "flyers" (that is, bullets which are
deflected from the normal path) caused by rain drops in the suppressor. The
wooden stock is impregnated with epoxy resin to make it warp-free in high or
low humidity conditions (a shrunken or swelled stock changes the rifle's zero).
New 1/2-minute rear sights replace the 1-minute of angle rear sights and the
front sight blade is reduced to 0.062 inch in width for long range shooting.
The front band is permanently attached to the gas cylinder. The barrel is modified
to prevent gas cylinder and operating rod sloppiness which would affect accuracy.
The operating rod spring guide is changed from a stamping to a tapered rod so
the spring will stack uniformly. The M-14 becomes an M-21 rifle with the addition
of a scope mount and an adjustable ranging telescopic (ART) sight or AN/PVS-2
or AN/PVS-4 starlight scopes.
The ART sight is a 3 to 9 power,
Parkerized steel, nitrogen-filled telescopic unit that has a ballistic cam matched
to the 7.62mm M118 Special Ball. The M118 Special Ball has a muzzle velocity
of 2,550 feet/second and a 173-grain, full metal jacket boat tailed bullet.
The M118 is effective out to 1,100 yards.
The Sionics suppressor was designed
and manufactured by Military Armament Company of Alpharetta, GA. The Sionics
suppressor replaces the standard M-14 flash suppressor. Once the M-14 flash
suppressor is removed, the Sionics suppressor nut and split bushing are installed
and suppressor is screwed onto the barrel. The purpose of the Sionics suppressor
is to moderate the muzzle blast of the rifle (to make it hard to locate) and
hide the muzzle flash at night. It is not a "silencer" in the classic
sense, but it so reduces the muzzle blast that the noise made by the rifle extracting,
ejecting, reloading, chambering, and locking is VERY apparent. The Sionics suppressor
has a bleed valve to exhaust the gas from the suppressor after firing.
Snipers or other users of the M-21
who were firing at night would fire one round into the ground and then apply
tape over the muzzle of the suppressor. The combustion gases would be trapped
and there would be no oxygen available to support a muzzle flash when the rifle
was fired in anger that night. Snipers would use the gas cutoff to make the
M-21 a manually-operated rifle to eliminate the now noisy semi-auto action.
M-21 maximum effective range with
the AN/PVS-2 or AN/PVS-4 starlight scopes is about 400 meters (440 yards). Maximum
effective range with the ART scope in daylight is 1,000 meters (1,100 yards).
The sight mount can accommodate either the ART or starlight scopes without any
loss of zero. Weight of the M-21 with ART scope, Sionics suppressor, and a 20-round
magazine is approximately 12.5 pounds. Changing from day to night optics adds
another 2.5 pounds to the weight.
The M-21 sniper rifle has been product
improved into the M-25 sniper rifle. The M-25 adds a receiver lug to further
minimize shifting in the stock. The scope mount is changed to a Brookfield Precision
5-point type; the inverted T-lug on the receiver (3 points), the charger guide
(1 point), the forward receiver ring (1 point). The ART scope is replaced by
a 10-power Leupold M3, M3A, or Mk4 scope with Mil-Dot reticle. Naval Special
Warfare units use both M-21 or M-25 sniper rifles today.
Photo: ATT Inc
A Sionics-type suppressor (assembled, background) and
its component parts (foreground). Installation was very simple
(after the frontsight/flash suppressor of the M-21 was removed.
The cap nut and split rubber bushing were removed from the
rifle and installed over its muzzle. The suppressor was screwed
onto the muzzle of the M-21 and the split rubber bushing
inserted into its base. The cap nut was screwed onto the rear
end of the suppressor to compress the rubber split bushing and
seal off any gas that might bleed back. This particular
suppressor is for the MAC-11 9mm submachine gun; the
suppressor for the M-21 is very similar except that it is longer.
Photo: Sniper Country
The new M-25 Sniper's rifle as used by today's Naval Special Warfare
units. Note the adjustable stock comb to help position the sniper's
eye directly behind the sight. The scope is a 10X Leupold Mk 4. This
camouflaged rifle is equipped with a Harris bipod.
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@ 2002 Bob Stoner