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40mm Bofors Automatic Gun M1 (AA) on M2A1
mount raised to near maximum elevation. |
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A dusting of fresh snow covers this Bofors
of the 532d AAA. The hood
is in place over the automatic loader
to protect it from the elements.
This crew will fire by sight control using
the M7 computing sight (Weissight). |
The 40mm Bofors antiaircraft gun was developed in the 1930s by
the Swedish company
of the same name, and rapidly became the firm's most successful
weapon design. The Bofors product was used by both Allied and
Axis forces in World War II and was the most common antiaircraft
artillery gun of the war. By the beginning of the global conflict, Bofors had sold their gun
(or granted production licenses for its
manufacture) to 29 countries.
The British had been using the Bofors since 1937, and it was
through their
assistance and influence that the 40mm was brought into the United States Armed Forces.
Although some in the Army may have been a little reluctant to look
at the Bofors, the general lack of confidence in the
37mm automatic gun opened the door to a fair
hearing for the Swedish weapon. The Bofors was impressive in
testing, so the Army standardized the foreign forty in May 1941,
modifying the gun's dimensions and tooling to American standards of measurement
in order to facilitate mass manufacturing in U.S. factories. This
design was dubbed the 40mm Automatic Gun M1 (AA).
The original M1
carriage for the gun was simply the stock Bofors mount. It was
quickly replaced by an improved American version classified as the
M2. The later M2A1 carriage differed from the M2 because of
its higher gear ratios in the traversing mechanism. This
change allowed the M2A1 mount to traverse the Bofors three times
farther in azimuth in a single turn of the lateral pointer's
handcrank when compared to the M2. The higher ratio of the
M2A1 helped the gun to track fast-flying aircraft, even if enemy
pilots dropped low and flew close-in. The rate of change in
elevation was the same for both the M2 and M2A1 models. All
carriages could be fired "from the wheels" when necessary.
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40mm gun being tested at Aberdeen Proving
Ground. Ordnance and Coast
Artillery brass were sufficiently impressed with the Bofors to
adopt it as a standard automatic weapon to
supplant the 37mm gun. |
An awkward early attempt at making the 40mm
air transportable.
The smaller, lighter M5 carriage was soon
developed
for use with airborne AAA units for easier
loading in
a C47 transport aircraft.
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40mm Bofors Antiaircraft Gun Facts
Firing
Table Muzzle Velocity: 2,800 feet/second
Breech:
Automatic
Maximum
Rate of Fire: 120 rounds/minute
Elevation
Limits: -5º
to 90º
Recoil
Type: Hydro-spring
Fire
Control Director: M5
Maximum
Effective Slant Range: 1,500 yards
Maximum
Effective Horizontal Range: 3,500 yards
Maximum
Effective Vertical Range: 3,500 yards
Maximum
Effective Fire Control Altitude: 7,500 feet
Actual muzzle velocity was dependent on ammunition
used and environmental conditions. Range was limited by tracer
burnout, at which point the projectile was destroyed. |
The Antiaircraft Artillery
40mm fire unit consisted of a range section, responsible for operating the
M5 director and power plant; and
the gun section, manning and serving the 40mm piece and its accompanying
machine gun(s). Each fire unit was commanded by a chief of section
(CS), who reported to the platoon commander. The gun section was
managed by a gunner (G) responsible for the proper emplacement of the guns
and their performance in action. The range section's leader was
called a range setter (RS), and he guided the setup of the director and
generator, verified the orientation and synchronization of the gun and
director, and supervised fire control either at the M5 director or by the
carriage when the M7 Weissight was used.
The range section was
manned by the range setter, elevation tracker, azimuth tracker, power
plant operator and telephone operator. The gun section included the
gunner, two machine gunners for the .50 caliber weapon, the vertical gun
pointer, a lateral gun pointer, an ammunition loader and firer, and two
ammunition handlers and relayers. This made for a total of fourteen
men. (There was only one soldier in the ammunition detail in a semimobile
automatic weapons unit, thereby reducing the section count by one man.)
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Equipment of the 40mm fire unit: Bofors
gun, director, director chest, cabling, generator and .50 caliber machine gun
on M3 mount.
Throughout the war, 40mm fire units
replaced the single water-cooled machine gun with the quad mount M51
trailer.
Antiaircraft units that abandoned the use
of the M5 director simplified their setup by eliminating all of the above,
except the guns.
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The 40mm Bofors was operated either by sight
control or by director control. It was possible to switch between
the methods as needed. If the director or generator failed or
was damaged, manual operation by sight control was the standard backup
procedure. The assigned duties of section personnel were
dictated by the method fire of control.
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Operating under sight
control, using any of the on-carriage sighting systems available for
the Bofors, the gunner took his position behind the 40mm gun and was
responsible for slewing the AA cannon toward the target. He
also would hold one end of a rope that was attached to the foot of
the soldier who fired the gun. A cease fire would be
ordered by pulling this rope. The vertical and lateral gun
pointers would sit in their respective seats on the carriage and
prepare to move the gun in azimuth and elevation by handcrank,
tracking the targeted aircraft through the gun sights. The
loader mounted the platform, kept the gun's autoloader fed, and
often fired the gun through means of a foot pedal. The
ammunition detail passed full 40mm clips to the ammo relayer, who
then passed them on to the loader.
The range section's
director trackers acted as lookouts in sight control, the machine gunners
manned their fifty, and the telephone operator maintained communications
or acted as a second cannoneer if a quad machine gun mount was in use.
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For director control, the
range section's elevation and azimuth trackers would spot the enemy
aircraft through the M5 director's two telescopes, as the range setter
slewed the director toward the target and adjusted the device's settings.
Once the director was tracking the airplane, the gunner on post at the
carriage would engage the slewing lever on the Bofors to allow the
movement of the gun to be controlled remotely by the M5 director.
The loader would be in place on the platform to supply the gun with
ammunition and to fire the weapon at the command "engage".
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Operating under director
control, the vertical and lateral gun pointers did not take their stations
on the carriage, since the M5 director was automatically aiming the gun in
firing elevation and azimuth as it computed the future position of the
enemy aircraft being tracked. Under director control, the gun
pointers maintained posts behind the gun and acted as lookouts.
The director was always
placed within 15 feet of the gun it controlled, creating a 70°
dead area behind the gun, a zone through which the Bofors could not fire.
This was a necessary safety precaution to protect the range section and
other firing unit personnel.
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40mm firing unit at the Tacloban airfield, Leyte, in October 1944.
The gun crew is preparing to take on
a ground target by sight control,
while a soldier from the range
section stands ready near the M5 director.
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In emergency situations, a minimum crew of four
could operate the 40mm by sight control. All members of the firing
unit were required to learn the combat duties of the lateral and vertical
gun pointers, the loader and the machine gunners. If disaster
struck, any three soldiers could mount the carriage to aim, load and fire
the gun. An additional soldier would be available to help slew the
gun, relay ammunition and, if needed, man the machine gun.
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Lateral gun pointer ready on his double
hand
crank. Cranking clockwise traverses
gun to the
left. The NCO on the left is slewing
the gun by lifting the power synchronizing handle and
swinging the piece. |
Another clip goes in the auto loader as
this crew prepares a practice fire.
With no gun pointers seated, this forty
will be fired by director control.
The Bofors could be laid remotely from the
M5 director, or could be
traversed and elevated manually by
pointers using on-carriage sights. |
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The Bofors used a standard high-explosive shell
with a sensitive point-detonating fuze designed to burst upon impact with
the relatively lighter materials used in aircraft construction. If
the shell did not make contact with a target, the fuze destroyed the
projectile at the tracer burnout range of approximately 3,500 yards.
Armor-piercing shot was also available to take on ground targets.
Each 40mm round weighed about two pounds.
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Loading a standard four round 40mm clip. |
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Other Bofors Users
The United States Navy used the
40mm Bofors,
most commonly in a side-by-side twin mount configuration. The Navy
also modified the standard Swedish designs and developed a quad 40mm under the Bofors manufacturing license.
In Germany, the 40mm Bofors gun
was called the Flak 36. It was not a common Flakartillerie
weapon for the Germans, and most of their forties were captured
Polish pieces. The Germans did field a few truck mounted Bofors.
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40mm Bofors shown
in towing position, complete with a spare tire mount. The
carriage featured four-wheel independent spring suspension and an
all-wheel electric brake system.
The Bofors would be towed in most cases by a 2-1/2 ton truck. |
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Stateside practice on the sands
of Sears
Landing, a Camp Davis firing range along the coast of North
Carolina.
The boxlike M5 director for this gun is
partially in view along the right edge of the photo, behind the lieutenant
and the soldier in denims.
The director is automatically aiming the
gun by remote control, as there are no gun pointers in position on the
carriage.
The loader is standing on the mount and has
his foot on the rear firing pedal. It is his responsibility to
trigger
the gun in this situation.
The gunner is standing behind the Bofors,
grasping a rope. When "cease fire" is ordered, or an unsafe situation develops,
the gunner
yanks the rope, pulling the loader's foot
off the firing pedal. A crude, but effective, system of
communication.
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FIRING THE FORTY
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Standing to the side and rear of the
vertical gun pointer,
the loader could fire the Bofors
using the rear pedal.
A rope is tied around his right foot,
resting on the pedal. |
The vertical gun pointer also had a firing
pedal. He could
trigger the weapon by simply depressing
with his right foot.
A clockwise crank on this side of the mount
elevates the gun. |
If the firing pedal remained depressed,
the gun would continue to fire at the full automatic rate of about 120
rounds per minute.
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