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Side view of the Combination
Gun Motor Carriage M15A1. |
M15 in camo paint. |
No Army commander doubted the
need for mobile antiaircraft protection for troops on the move.
Several experimental or improvised mobile mounts had been developed for AA
machine guns before the standardization of the M13 and M16 multiple
machine
gun halftracks. However, America's rapidly expanding armored force
was seeking a highly mobile antiaircraft mount that could provide more than
a machine gun umbrella.
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T28E1 halftrack, the forerunner of the M15,
with its water-cooled machine guns. |
Since the Antiaircraft
Artillery was in the process of replacing their towed
37mm automatic weapons
with the recently standardized 40mm Bofors gun, it
seemed
obvious to attempt to utilize the
37mm gun in some form. The M3E1 coaxial mount
that teamed a 37mm with two water-cooled .50 caliber machine guns showed
promise. A mount of this type was soon married to a halftrack
chassis. A later variant of this new test motor carriage made an immediate positive
contribution on its first missions under actual combat conditions. An officer observing Allied
landings in North Africa was impressed by the experimental
T28E1's performance, lauding
the halftrack's fire as "devastatingly effective against low-flying
aircraft." However, the Army was already sold on the self-propelled
mount and had refined the halftrack into the M15, which was standardized in November 1942.
In the final design, armor was added and the water-cooled machine guns were dropped in favor
of heavy barrel Browning M2 fifties. Production began
in earnest, and the Antiaircraft Artillery began reorganizing some
existing automatic weapons units into self-propelled battalions to
highlight
the effective new weapon.
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The coaxial mounting of a
37mm with the machine guns actually helped improve gunnery.
Technically, the tracer stream from the twin fifties provided gunners with
improved fire control for their 37mm canon, which had always been a
concern. Tactically, clever M15 crews often greeted approaching
enemy aircraft with only machine gun fire, waiting to open up with their
bigger gun. Emboldened by the belief that he was opposed by a
machine gun position, many an Axis pilot risked flying just a little
outside effective machine gun range, only to find himself suddenly and
inescapably caught in the longer reach of the 37mm. The ploy often
worked, damaging or downing numerous enemy airplanes.
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Halftrack crew "in the bucket" near
Cherbourg, France.
One
foot pedal fired the .50 caliber machine guns; another the 37mm gun.
There are five soldiers in this photo. Can you spot them all?
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The M15A1 was the more
plentiful upgrade on the M15 design and was built on the improved M3A1
halftrack chassis. It sported a lower
profile that brought the top edge of a slightly narrower turret about
a foot closer to the
ground. Other modifications were incorporated to better accommodate
the crew and provide improved clearances for the guns. The M15A1 weighed about 2,400 pounds less than the M15
and was believed to be a more stable carriage. A general rule of
thumb to distinguish the variants is to observe which gun mount is used on
the halftrack. The M15 mounts the twin machine guns above the 37mm
(M42 mount),
while the M15A1 had the fifties fixed slightly below the barrel of the larger gun (M54 mount).
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A comparison between the limited standard M15 (left) and
standard M15A1 (right).
Under the hood was a conventional White
in-line six cylinder gasoline engine, giving the halftracks
a top speed of 45 miles per hour on level
ground.
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Top view of an M15.
Note the rail above and behind the
driver's compartment. This kept the guns from depressing into the
compartment. |
The M15 and M15A1
halftracks were manned by a crew of seven soldiers. The squad leader
commanded a driver, gunner (also know as a lead adjuster), two cannoneers,
and the lateral and vertical gun pointers. When mounted, the squad
leader and driver rode up front. The remaining five soldiers were in
the bucket with the guns.
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M15 with armor in place. 1/4-inch
steel shutters protect the radiator. 1/2-inch plate covers the
windshield opening. The entire driver's compartment could be
enclosed. This armor was common to the M3 halftrack family. |
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Detail of the vertical pointer side of the
turret. The left foot fired the
37mm gun. The side of the foot pushed a lever, unlocking the pedal safety. Both machine guns
were fired by the foot treadle above the right foot rest. |
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Field fortifications
for the M15 halftracks consisted of pit with a level bottom and
sloped ramp (as illustrated above). Ideally, the pit would be
deep enough to protect the halftrack but not impede horizontal
firing. In less than optimal conditions, the halftracks were
partially dug-in and sandbagged (as shown in the photo to the left).
Where a rapidly changing situation did not afford the time required
to dig-in, a few sandbags and some camo netting would often suffice.
Fortifications
would be the same for any series AAA halftrack. |
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(Left) M15A1
halftrack of the 834th AAA AW Bn (SP) beachside, protecting the right
flank of III Marine Corps in the Pacific.
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Why Not a Halftrack Mounted 40mm?
If the 37mm gun was so successful on a halftrack, why wasn't the
same concept attempted using a 40mm Bofors? Actually, it was.
As early as 1941, the Army briefly toyed with a 40mm gun mounted on
a burly three-quarter track manufactured by Mack. This
carriage was large enough to platform the gun, its director and
power plant. The design was not adopted. Several other
test configurations based on the sleeker M3 White halftrack were
evaluated, including the T54 (pictured here). None were
standardized.
Shortly after the Normandy Invasion, the Antiaircraft Artillery
Board did standardize a mobile twin 40mm rig that utilized the M24
light tank chassis and a pair of Bofors in a turret. However,
this new M19 motor carriage did not replace the M15 halftrack in
action before the end of the war, as intended.
Incidentally, other forces that used the Bofors did utilize the gun
on truck mounts, including the Free French and the Germans.
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Non-standard
equipment improvised in the field was sometimes employed by
isolated U.S. Army AAA units. If the halftrack pictured
at the left looks like a normal M15, don't be fooled.
That is really a 40mm Bofors in the bucket. A group of
40mm halftracks was in service with the 209th AAA AW Bn in the Pacific. These special
tracks were modified in Australia and used by that organization with
positive results. American ingenuity was always at work throughout World War
II. |
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Fifth Army antiaircraft artillerymen
prepare to take on the Luftwaffe in Italy with their M15's combination gun.
The soldier on the lower left, peering
through a scope, is the vertical pointer. On the other side of the
gunner (who is wearing goggles to aid in seeing tracers), is the lateral pointer, also looking
through a single-power telescope. These soldiers track the target in elevation
(angular height) and azimuth (compass direction), moving the gun mount
with hand wheel cranks. The gunner
observes the tracer stream, applying any necessary corrections by turning
a deflection handle, which he is gripping with his left hand. The
soldier standing to the side of the mount is waiting with another 10-round
feeder clip for the 37mm. Teamwork is critical!
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Rear view of the bucket that shows the
M6 sighting system of the M42 mount. The gunner's L-shaped deflection handle is
clearly seen midway between the spotting scopes on the mounting bar.
The vertical pointer's hand wheel is visible on the right side of the
mount,
between the seat and the machine gun's
"tombstone" ammo chest.
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