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Ordnance Notes --
by Bob Stoner GMCM(SW) Ret.
M-40A1 106MM Recoilless Rifle With M-8C Spotting Rifle
(See photos at
bottom of page)
The 106mm recoilless rifle M-40A1 and
its M-8C .50 caliber spotting rifle are now limited standard items with
the U.S. armed forces. The M-40 is the largest in a long line of
recoilless rifles beginning with the 57mm M-18 and M-20 75mm recoilless
rifles developed at the end of World War 2.
A recoilless rifle or RCL has a rifled
barrel, unlike the smoothbore rocket launchers of the WW2 era. That is,
the 2.36-inch rocket launchers M-1 and M-9 used fin-stabilized, solid
propellant rockets. What differs with the RCL is its ammunition looks
like a conventional round of fixed ammunition, but with the case
resembling a Swiss cheese with all its vent holes.
Like the rocket launcher, the
recoilless rifle can be lightly built due to its method of operation. The
RCL vents an equal mass of propellant gases to its rear as its projectile
exits its front. (This makes it a huge propellant hog.) It differs from
the rocket launcher in that it fires a shell (with a pre-engraved rifling
band) instead of a spin-stabilized rocket. Holes are formed in the case
of the recoilless rifle cartridge. The holes allow the propellant gases
to escape and exit to the atmosphere through vents in the rifle's breech
block. When the mass of the gases being exhausted to the atmosphere equal
the mass of the projectile being fired, the launcher or rifle is
"recoilless."
Depending on its size and caliber, the
recoilless rifle may be fired from the shoulder or from a mounting. The
57mm RCL M-18/M-18A1 can be fired from the shoulder for from the
M-1917A1/M-74 machinegun tripods. The 75mm RCL M-20 is fired from the
M-1917A1/M-74 machinegun tripods. The 90mm RCL M-67 is fired from the
shoulder. The 105mm RCL M-27 is fired from a permanently-attached
vehicular mounting. The 106mm RCL M-40A1 is fired from an attached
mono-wheeled M-79 tripod.
The 106mm RCL was designed as a light
weight anti-tank weapon that could kill main battle tanks. Ammunition was
high explosive anti-tank (HEAT), high explosive, plastic (HEP),
anti-personnel (APERS), and drill (inert). The .50 caliber spotting rifle
M-8C was designed to be the primary ranging device for the weapon. The
.50 caliber spotter round was much shorter than the .50 caliber Browning
machinegun cartridge. The spotter round was a ballistic match to the HEAT
and HEP rounds fired by the M-40A1. On impart with the armored target,
the gunner saw a puff of white smoke in his sight. If the puff of smoke
was on target, he fired the main gun. If not, he made his traverse and
elevation corrections and fired the .50 spotter again. A one-shot hit
with the 106mm was imperative, because the back blast from the breech
extended in a cone-shaped fan 75 yards deep and 150 yards wide. A tank
would have to be both stupid and blind to allow the 106mm gunner a second
shot under such conditions.
HEAT round. The HEAT round uses the
Monroe-effect to penetrate armor. The cone-shaped nose of the HEAT round
is hollow. The hollow inside of the round is roughly diamond shaped; the
upper half of the diamond is the sheet metal nose and the bottom is a
copper cone. The empty space in the round forms what is known as the
"standoff distance" and the copper cone focuses the blast which punches a
hole through the armor plate. The explosive mixture is located in the
body of the shell behind the copper cone and is capped with a
base-detonating fuze. When the HEAT round hits armor, the nose of the
projectile crumples until the cone comes in contact with the armor.
Milliseconds after the hit, the base detonating fuze fires to detonate the
explosive. The copper cone focuses the energy of the blast on a small
spot of armor and burns a hole in it; the copper cone is formed into a
slug which is blown through the hole into the tank. Armor hit by the HEAT
round shows a characteristic circular scorch mark with a hole in the
center similar to that caused by an acetylene torch.
HEP round. The projectile of the HEP
round is more streamlined and rounded than the conical-shaped nose of the
HEAT round. The HEP round operates against armor in a less spectacular,
but most efficient way. The HEP shell (called "Squash Head" by the
British) is actually a mesh bag filled with explosive inside a thin
skinned projectile. When the HEP shell hits the armor, the projectile
skin breaks away and the explosive-filled mesh bag mushrooms out against
the plate. Milliseconds after the hit, a base detonating fuze explodes
the filler. The explosion sends a shock wave through the armor plate that
breaks off a huge chunk on the opposite side of the plate and sends it
bouncing around the inside of the tank at very high velocity. Armor hit
by HEP exhibits a characteristic circular scorch mark on the outside and a
huge crater (where the armor was blown away) on the inside.
APERS round. The APERS round converts
the RCL into a giant shotgun for use against infantry. The projectile is
filled with 6,000 13-grain flechettes (looking like nails with fins)
stacked nose-to-tail. The APERS round resembles the HEP round, but it has
a nose fuze that allows it to function at the muzzle or at a preset
distance from the muzzle of the RCL. The APERS round also has a tracer
element in the base of the projectile.
DRILL round. The DRILL round is an
inert cartridge that simulates the size and weight of the HEP round. It
is used for training.
The M-40A1 is a crew-served weapon;
that is, more than one man is required to operate it efficiently.
However, one man can also do it at a reduced rate of fire. The following
procedure assumes that the M-40A1 and M-8C have both been bore sighted
with the main optical sight.
The loader pulls down the breech
handle and swings open the breech block. The loader inserts the shell and
indexes its pre-engraved rifling with the rifling in the gun barrel. The
loader closes the breech block and positions himself forward and to the
right of the breech.
The gunner sits on the tripod leg and
acquires the target through his optical sight. He tracks the target using
a traverse wheel mounted horizontally on the tripod base. For fast moving
targets he can declutch the traverse wheel and move the gun with his
body. He adjusts elevation by a vertical hand wheel on the gun mount.
There is a mushroom-shaped button in the center of the elevation wheel.
When it is pulled OUT the .50 caliber spotting rifle fires. (The .50
caliber spotting rifle is semi-automatic and fires from a 10- or 20-round
magazine.) When the button is pushed IN the 106mm round is fired.
The gunner uses the hand wheels to put
the gun on target. He ranges the target with the .50 spotter rifle whose
ballistics are the same as the 106mm round. He follows the .50 spotter
round to the target by following its tracer trail and notes the location
of the white smoke puff. If on target, he fires the main gun. If not, he
continues ranging until he is on target.
The weight of the complete M-40A1
weapons system is approximately 461 pounds. It has an overall length of
11.2 feet and a maximum range of 8,420 yards. Muzzle velocity of the
round is approximately 1,650 feet/second. Armor penetration is
approximately 6 inches at 60 degrees to the vertical.
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A model of the
M-40A1/M-8C as seen from the right and rear. The breech mechanism is
clearly shown, as are the hand wheels, sights (above the spotting
rifle), and the mounting of the spotting rifle. The two black cables
are the firing devices for the spotting rifle and the main gun. Also
shown are the hipping tubes for the ammunition and replicas of the
specialized subcaliber practice rounds used for training. These rounds
used a fired 106mm case fitted with a gun barrel and bore guide at the
muzzle. They were used to train crews with small caliber ammunition on
restricted ranges where the 106mm round could not be used.
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