Duster's Last Stand: The M42 Anti Aircraft Weapon System’s Final Days

By Robert Bruce

The 40mm M2 Dual Automatic Gun is a truly awesome weapon when its twin machine cannons are pumping out 2-pound high-explosive tracer projectiles at the rate of four rounds each second to a maximum range in excess of 10,000 yards! On US Navy ships throughout World War II and on US Army armored vehicles in the latter part of the war and during Korea and Vietnam, the range and power of these Swedish-designed BOFORS guns was bad news not only to propeller-driven fixed wing aircraft, but to helicopters, armored vehicle convoys and even fortifications. In the broad category of “machine guns”, this is about as big and bad as the come.

What follows is not only a close look at the M2 in its land warfare configuration, but also an account of the last day the 40mm system was fired in the Virginia Army National Guard - among the last components of the United States Army to utilize the M42A1 “DUSTER.” On 14 April 1987, the Tank Range at Fort Pickett, Virginia vibrated with the measured thumping of eight BOFORS guns on four big, heavy, fully tracked and armored DUSTERS firing their last rounds.

The well-respected Swedish artillery manufacturer BOFORS introduced an automatic-loading 40mm antiaircraft cannon in 1929. This superior design found a receptive export market for defense of ships and ground targets. By the end of the first year of World War II, it was in first line service with the British and American military and sixteen other major nations.

Its performance and shell weight made it ideal for dealing with low-flying aircraft; it was tough, simple, and reliable. The key to its mechanical performance was the top-mounted “autoloader unit.” This allowed a four round clip of cartridges to be dropped in place and gravity-fed into the gun as the breechblock recoiled open and slammed closed by spring/hydraulic mechanism. Although the practical rate of fire was some 60 rounds per minute, in a dire emergency, 120 rounds per minute could be fired without interruption by continuously dropping in fresh clips.

The American Army began producing it under license in 1941 as the 40mm Automatic Gun M1, and by the end of World War II some 34,000 had been turned out. The gun was well regarded on its initial M2A1 wheeled mount, easily towed behind trucks and quickly set up. However, the rapid movement of American armored divisions urgently necessitated development of a mobile gun carriage that would allow instant engagement of hostile aircraft.

The solution was to mount a twin Bofors gun atop a tank chassis for exceptional cross-country mobility. The cannon crew rode right in the gun tub so that they could immediately fire when their armored column was attacked by LUFTWAFFE fighters. Thick armored shields protected them from the attacking aircraft’s machine guns. The M13 Computing Sight took much of the guesswork out of the tough job of tracking a quick and nimble fighter. A power-operated traversing and elevating system turned the heavy gun and mount quickly and smoothly with a twist of the gunner’s wrist.

The fully tracked and armored M19 Motor Gun Carriage, with its twin 40mm BOFORS M2 Dual Automatic Machine Cannon, was adopted in June 1944. This clever combination of a modified M24 CHAFFEE Light Tank chassis and the combat-proven, quick-firing M1 heavy cannon provided the US Army’s fast-moving armored forces with an extremely potent weapon in the closing months of World War II.

In the Korean War of 1950-53, the M19 proved no match for the jet-powered MiG 15. Despite the powered turret and relatively sophisticated fire control of the M19, the Russian-built (and flown) jets were just too fast. It did, however, find excellent employment against enemy earthworks and vehicle concentrations.

Wearing out quickly in the harsh conditions of warfare on the Korean peninsula, the M19 was replaced in October 1953 by the M42. This improved system used the same gun and fire control, now mounted on the M41 “WALKER BULLDOG” tank chassis. Its intended post-war role was to keep up with fast-moving M48 “PATTON” Medium Tanks and provide them with protection against enemy rotary-wing aircraft.

Later, with the introduction of a radar-directed 20mm VULCAN powered cannon and rapid developments in antiaircraft missile systems, the M42 was clearly obsolete. However, its impending demise was delayed when the US began to take over defense of South Vietnam. This was a war in which the enemy had no aircraft in the South, but a lot of ground troops. The “DUSTER” found itself once again in the utilitarian role of direct fire against fortifications and even massed troops.

Although officially declared obsolete by the US Army after the Vietnam War, the M42 soldiered on in the National Guard until 1987 when the far more efficient STINGER anti-aircraft missile system.

The M42A1 is a full-tracked, armored, antiaircraft vehicle that can operate over all terrain compatible with medium tanks. Major armament is the 40mm dual automatic gun M2A1 on the M4E1 mount. This vehicle was designed for use with the maneuver forces against low-altitude air attack. Because of the rapid rates of fire, the gun proved to be effective as a support weapon against ground targets.

The interior of the vehicle is divided into three areas: driving compartment at the front, stowage compartment in the center, and engine compartment in the rear. The driving compartment contains the driving controls and instruments, as well as seats for the driver and the commander/radio operator. The stowage compartment serves as a base for the gun mount and has space for twelve boxes of 40mm ammunition. The engine compartment houses the main engine, auxiliary generator and engine, transmission, and fuel tanks. The gunner, squad leader, and cannoneer ride in the gun mount. The squad leader commands, using the intercommunications set AN/VIC-1, either from the commanders seat in the driving compartment or the gun mount.

Crew

This official Army doctrine was modified somewhat in practice. As with traditional artillery crews, each man is given a number as shown on the accompanying diagram. A National Guard “DUSTER” crew consists of a Squad Leader (SL), #1 Gun Pointer, #2 Right Cannoneer, #3 Left Cannoneer, and #4 Driver. Specific duties are assigned to each not only when firing, but also in routine operation and maintenance of the vehicle, gun, and mount.

The Squad Leader gives all gun commands. He is responsible for the efficient operation of the crew and system, as well as giving accurate fire control information on the intended target. The Gun Pointer actually aims the gun using the power control system or the hand wheels. He aligns the gun on target using either the direct fire ring sight, the direct fire M38 computing sight, or indirect fire with the azimuth indicator and gunners quadrant. He also fires the guns on command of the Squad Leader.

The Right and Left Cannoneers are more accurately termed “loaders” when the gun is actually operating. In addition to performing a specified series of mechanical tasks on each gun in immediate preparation for firing, these men lift the heavy clips of ammunition into place on the cannon. At the command “CEASE FIRING AND SECURE,” they unload any remaining rounds, open and inspect the breech, and set the safety lever. Their job becomes very hectic when reloading in sustained fire and quite dangerous when quickly removing misfired shells that may “hang-fire” at any moment! They are also responsible for the cleaning and lubricating of their cannons - a hard chore under any conditions.

The Driver is responsible for all the mechanical aspects of the vehicle, engine and track system. In additon to being skilled at maneuvering it cross-country, he must be able to perform routine repairs, and to ensure proper lubrication and other operator-level maintenance. All members of the crew are cross-trained in basic duties of the others so that the fire mission can be carried out even when there are casualties or shortages.

Loading and Firing:

Left and Right Cannoneers each pull their hand-operating lever all the way to the rear and engage it in the rear latch bracket. Set the fire selector lever in the SAFE position. Push a clip of ammunition down the guides of the loader until the feed rollers are rotated and a cartridge drops into the loader tray. Immediately before firing is to take place, they disengage the hand-operating lever from the rear bracket latch and swing it forward to lock into the front latch bracket. Set the firing selector lever as ordered by the Squad Leader or Gunner to SINGLE or AUTO FIRE.

On the Squad Leader’s command to FIRE, the Gunner depresses the foot pedal to release the breechblock on the left gun. This allows it to run forward under spring and hydraulic pressure to feed, chamber, and lock a round. The firing pin trips automatically on locking, detonating the round.

The accompanying diagrams, taken from the official Army Technical Manual on the 40mm Automatic Gun M1, show the cycle of operation. On the twin mounted M2A1 Dual Automatic Gun a mechanical linkage ensures that the left gun must fire and recoil before the right gun can fire. This alternating feature gives a smooth, continuous action. In the event of a malfunction in one gun, this can be overridden so that at least the other gun can fire while the problem is being worked on.

Aerial Targets

The simplest form of aiming is provided by the Speed Ring Sight (8), a metal spider web that allows a trained gunner to mentally compute the speed, range, and attack angle of an aircraft, then “hold-off” by placing it in approximately the right ring while shooting. This allows some degree of accuracy in “panic shooting” at an airplane or helicopter that comes on suddenly. Tracers help “walk” the burst into the still moving target.

Whenever possible, the M38 Computing Sight should be used to engage aerial targets. This device automatically provides the gunner with the correct “lead” to the target. The accuracy of this is largely dependent on the skill of the Squad Leader (1) who must estimate the target speed, direction of flight, and the angle of dive or climb (2). He quickly sets these values into the M38 (8) while the Gunner (3) engages the power drive mechanism and aims the guns (4) in the direction of the target until it is centered on the reticle of the M24C Reflex Sight (5).

The M38 system mechanically computes the lead angle (6) based on target speed and flight direction; extra elevation and traverse is automatically inserted. When target tracking is steady, the Gunner reports “ON.” If the target is still within range, the Squad Leader commands “FIRE.” The Gunner depresses the foot pedal to fire the guns and continues firing until the target is destroyed or the Squad Leader commands “CEASE FIRING.”

Ground Targets

The Speed Ring is most useful in engaging moving targets on the ground such as trucks and armored fighting vehicles. With only range, speed and left-right movement to contend with, even a novice Gunner can quickly “walk” a burst right into the target with a minimum of wasted rounds. Also, the potent twin 40’s can be used as conventional artillery for indirect fire (out of the line of sight of the crew) by laying the guns for direction and elevation figures provided by a fire control officer. This exercise is made quite precise by use of the on-board Azimuth Indicator and the hand-held Gunners Quadrant.

Ammunition

It is important to choose the right ammunition for the job at hand. Most antiaircraft work is done with HIGH EXPLOSIVE-TRACER (SELF-DESTROYING). This has a muzzle velocity of 2,870 feet per second and a maximum vertical range of some 7,625 yards, but the tracer burns out at approx. 5,000 yards. It also features an internal time fuze that causes the shell to self-destruct near the limit of its maximum range if it misses the target.

Anti-armor work calls for ARMOR-PIERCING-TRACER. While its penetration of steel plate is not sufficient for use against main battle tanks, it will still punch right through most of the world’s infantry fighting vehicles and armored personnel carriers. This round is also just the ticket for busting up the enemy’s hardened concrete bunkers or even deep earthworks. This is the ammunition being fired by the crews in this feature.

Causing havoc among enemy supply truck columns or urban fighting positions calls for HIGH EXPLOSIVE INCINDIARY-TRACER. This not only has a nasty explosive effect, but the incendiary component will set just about anything except water or dirt on fire. Imagine the effect of a 20-second burst (80 rounds) of HEI-T on enemy rebels holed up in an abandoned hotel in some hotly disputed Third World city. No wonder the M42 is still in first line use with a number of South American and Asian countries!

Shooting the M42

The DUSTER is not the sort of weapon system that is likely to be encountered on the modern battlefield. A mere 5-second burst hurls 20 rounds - over 40 pounds of death and destruction! It would take a truckload of 7.62mm (.30 caliber) or 12.7mm (.50 cal.) machine gun ammo to equal the projectile weight — much less the explosive effect and resultant devastation.

At the time of this activity I was a Sergeant First Class assigned to the Headquarters of the Virginia Army National Guard, and it had been my privilege to attend numerous live fire training exercises conducted by the Guard. The last range day for the M42 DUSTER was one that I was not going to miss, so I set off for Fort Pickett for a look at history being made. I joined the soldiers of the 3rd Battalion of the 111th Field Artillery as they said a fond farewell to their faithful machines in a day of really big bore ammo burning and target punching.

The unofficial orders had come down from the chain of command — “Unless you shoot up all the obsolete 40mm ammo in storage at Pickett, it will have to be demilled at no small cost to the taxpayers.” The “Redlegs” of the 111th were determined not the let such a terrible fate befall a mountain of perfectly good 40mm ammo. It was their duty to pump it downrange as a last and final salute to the combat-proven DUSTER.

Prepare to Fire

A tractor-trailer load of ammunition canisters was brought to the range and all soldiers pitched in to offload it and divide it up among the four gun systems. It takes two strong men to carry each 115-pound can of 16 cartridges. The M42 drivers, having completed after-operation maintenance checks on their vehicles, set about the task of unpacking the cans, and in some cases where the cartridges were bulk-packed, loading the nearly 5-pound cartridges into clips of four. Assisted by the Squad Leaders and Gunners, they lifted the 20-pound clipped ammunition onto the vehicle where it was stored in “ready racks” around the perimeter of the turret.

Meanwhile, the Cannoneers were carefully checking their guns and preparing them for firing. Numerous safety inspections and function checks must be performed exactly according to standards set forth in the Army Technical Manual that governs use of the gun system. These are well-tried and logical actions that ensure maximum safety to the crew, maximum efficiency in operation, and minimum chance for stoppages and malfunctions.

Aligning the Sights

Then, under the practiced and expert eye of the Squad Leader, “Bore sighting and Synchronizing” was carried out. “Synchronizing” is the process of adjusting the computing sight so the computer leveling pads are always parallel to the gun mount carriage leveling pads at any elevation of the dual gun.

Bore sighting is the process of aligning the lines of sight of the sighting instruments with a line of sight through the axis of the gun bores to a common aiming point no less than 1,000 yards distant. This becomes necessary when the vehicle has been in rough terrain operation for extended periods of time (heavy impact and vibration tends to knock things out of alignment). While this is not absolutely necessary under intense combat operations involving movement, it is desirable and certainly most efficient when the vehicle is on a range to conduct target practice that will be scored.

The small, gasoline-powered auxiliary engines were started, turning the electrical generators necessary for sustained power to the electro-hydraulic turret and other systems. When all manual and power checks had been completed for all four guns (and a number of hasty repairs and adjustments carried out by the unit’s skilled Armorers) the Range Control Officer declared the firing line “HOT” and the four guns on line were ready to rock ‘n roll at a series of ground targets.

“The Firing Line is No Longer Clear — COMMENCE FIRING!”

Within seconds the first short bursts were pounding downrange; blazing red-orange tracers marking their flight from muzzle to target. Spurts of propellant smoke issued from the tubes, clouds of trail dust lifted up from the tank engine, and - despite ear plugs worn by all - the sharp and heavy “POM-POM” sound of the alternating guns assaulted the hearing of crewmen and observers.

More guns joined in with the tempo and duration of the rhythmic pounding rising and falling as fire was adjusted and the tracers began finding their ground targets at varying ranges. Vapors of burning propellant drifted from the turrets, processed by one’s sense of smell as an odd combination of mild acid with a hint of perfume.

This preliminary firing exercise was designed to fully test all the basic functions of the gun and fire control system. It included engagement of prominent earthen mounds next to large wooden marker panels set off at predetermined ranges. Enormous painted numbers on each one revealed its distance from the firing line.

Four Graded Tests

The first test was, of course, actual functioning of the guns when firing bursts. One gun, despite having passed all preliminary checks, simply refused to fire a chambered round and required additional attention of the crew and Armorer. This failure was met with catcalls of derision from the other crews as they went on to methodically fire the next exercises.

Secondly, sight computation and bore sight validation was verified by “dialing up” a predetermined range on the M38 and having the gunner align the reflex sight reticle with the center of the earth mound next to the correct panel for that range. A short burst instantly reveals whether the procedures have been correctly done. Any significant problems become increasingly evident as the range increases from 800 to 1200 meters.

Inevitably, there are small deviations in elevation and traverse necessary to hit the mound squarely at the farthest range. These are set into the sight and the firing can continue. Large deviations are more of a problem, and usually signal equipment failure if bore sighting and synchronizing had been carried out correctly.

With direct line of fire to target confirmed, the third exercise is to power traverse left to right and right to left while firing on long linear walls of earth, set parallel to the firing line at a distance of 600 meters. If the gunner and system are working well, the effect is a neat series of dust splashes along the earth wall at regular distances and in approximately a straight line.

The final ground exercise is fired on the diagonal earthen wall, running across the left half of the range fan from 600 meters on the left to some 1,000 meters at its right extreme. As best it can be done on the ground, this is intended to represent the approach or retreat of an attacking aircraft. Hitting it evenly and regularly as the gunner simultaneously depresses the tubes while traversing right to left develops skill in tracking an incoming plane or helicopter. Elevating the tubes while traversing left to right mimics a retreating aircraft.

This last exercise is the most challenging for a gunner and it requires a lot of experience to be successfully done. Not only is there disruption in the aim point from rocking of the tank chassis under pounding recoil, but also the cloud of smoke from shells being fired tends to obscure the sights at critical moments. And, as I found out when it was my turn, the powered traverse and elevation is not as smooth, stable, and speed stabilized as it appears to the observer. Assigned gunners must get used to the quirks and characteristics of their system in order to be really good.

Despite the fact that this was the last live fire to be conducted before turning in their veteran machines, the range exercise was conducted with great seriousness and attention to the purpose of training gunners and crewmen. As usual, scorers were on hand to judge each crew on the full range of their duties including speed of gun drill, safety procedures, precision accuracy, and numbers of stoppages. There were trophies and prizes awaiting the best gunners and best overall crews. Good units with motivated and professional soldiers like to be able to measure their skills in fair and tough competition.

Cross-Training

With gun barrels dangerously hot from sustained firing, but plenty of ammunition to spare, the assigned gunners accepted their scorecards from the evaluators and made way for others in the crew to do the aiming and shooting. This was a rare opportunity for Drivers and Cannoneers to take charge of the gun systems and try their skill, and to suffer the mostly good-natured criticism of their teammates.

A 30-minute break in firing allowed the gun barrels to cool and for more ammunition to be placed in clips while system checks and adjustments were made. This was followed by another series of firing exercises - also graded - by much less experienced shooters including myself.

“Mad Minute”

A couple of hours later, when all assigned crewmen had fired a full set of exercises, there was still enough ammunition left for a “Mad Minute.” This is a spectacular event calling for all guns on line to fire as fast as they can for a full minute. This simulates what happens when the desperate call for “Final Protective Fire” comes as a defensive position is about to be overrun by the enemy. “Mad Minute” firing has become almost extinct as money for ammunition becomes increasingly scarce and only the most stingy live fire practice takes place. For safety purposes, only thirty seconds - 120 rounds - of continuous fire was to be allowed. This avoided the dangerous situation of a severely overheated gun “cooking off” and detonating a partially chambered round if one of the paired guns somehow failed to completely cycle.

Each DUSTER had 30 clips of ammunition stowed in the ready racks, on the engine deck, and stacked alongside on the ground. The first crew to fire all 120 rounds would win the “Mad Minute” trophy - the last to ever be awarded in the Virginia National Guard, and possibly the last for the whole United States Army.

Each Squad Leader in turn gave the READY signal and, after a moment’s pause, the Range Safety Officer blew the whistle to COMMENCE FIRING. With a second or two delay as the first clips were seated on the autoloaders and the guns un-safetied, all four DUSTERS erupted at nearly the same moment, “POM-POMMING” 40mm Target Practice Tracer rounds downrange. In another couple of seconds, clouds of smoke nearly obscured the entire firing line, but the relentless pounding continued. All of the guns were working at highest efficiency and the crews eagerly hefted clips of ammunition up to the turrets. Cannoneers slammed them one after another into the ammunition guides where they were quickly ingested into the smoking chambers of the BOFORS guns.

Downrange, the dirt walls and mounds seemed to move like a storm-tossed brown and green sea as tracers cris-crossed the front and each impact tossed plumes of dirt into the air. Numerous ricochets zoomed up and out at crazy angles, some tumbling end-over-end like pinwheel fireworks. The effect was truly awesome, serving as both an exhilarating and sobering reminder of the deadly power of the quick-firing 40mm automatic cannon.

After what seemed like much longer than the mere 37.6 seconds the official timekeeper later noted, one gun ceased firing and its crew jubilantly waived the green ALL CLEAR flag, indicating that they were the first to fire all 120 rounds. Slower crews, barely noting this and intent on the task, continued to lift, feed, and fire until at last all had completed the task. The last round thumped downrange and its sound reverberated against the distant wood line and then died away. Forever.

R-CAT Aerial Gunnery

In the early days of training anti-aircraft gunners, brave pilots would go aloft towing cloth banners from cables far behind their planes. After making a pass at the designated altitude, speed and direction — where a specific machine gun or cannon crew would try to hit the banner. Circling around, the pilot would drop the banner for the Gunnery Officer to count the holes and award this score to the crew. A second plane would tow another target over while the first would land to have a fresh one attached. This was inefficient, costly, and above all highly dangerous for the pilot and his craft when inexperienced or excited crews would “lead” the towed banner too much.

Interestingly, the solution to all these problems came from the radio-controlled aircraft hobby. It wasn’t long before the technique of using large ground-controlled model planes to tow scaled-down banners was tried and proved far more practical and safe. Each battalion of DUSTERS had a small R-CAT (Radio-Controlled Aerial Target) Detachment whose enviable job consisted of maintaining and flying six foot wingspan wood and cloth model planes powered by modified high-performance lawnmower engines.

Unfortunately for the realism of our final day of DUSTER gunnery, the R-CAT Detachment was not available to fly their mini-fighters. They had been ordered to prepare all of their planes and other equipment for immediate turn-in and orders - no matter how seemingly stupid - are not to be taken lightly.

STINGER

And in the days that followed, a 25-ton vehicle with a crew of five men was replaced by a shoulder-fired weapon requiring only a single soldier. The burden of anti-aircraft duties was officially surrendered to the high-technology STINGER missile system, a 34.5-pound supersonic “fire-and-forget” heat-seeking guided missile with the ability to engage aircraft approaching from any direction including head-on. As repeatedly proven in Afghanistan, its speed, long range, flight tracking and exceptional infrared counter-counter measures make the STINGER the equal of the most sophisticated ground attack aircraft.

Brute strength had been pushed aside by scientific triumph in a story as old as that of the crossbow replacing the broadsword. Although another sad day for those of us who love the smell of burning powder and the sound of a fully tracked land ship speeding over broken terrain, we take heart that the United States Army will continue to field the best weapons that our political leadership will allow us to buy...

Technical Specifications

1. General Data for Twin 40mm Gun Motor Carriage M42A1

Manufacturer:
   General Motors Corporation
Armament:
   one 40mm Automatic Dual Gun and one 7.62x51mm Machine Gun.
Engine:
   Continental Mfg., 6- cylinder, air-cooled, gasoline powered.
Transmission:
   Allison Mfg., CD500-3 Automatic, cross-drive.
Weight:
   49,500 pounds, combat loaded.
Crew:
   Driver + four.

2. Vehicle (modified M41 Light Tank chassis)

Length:
   251 inches.
Width:
   127 inches.
Height:
   113 inches, travel mode.
Max. Allowable Speed:
   45 miles per hour.
Fuel Tank Capacity:
   140 gallons.
Average Cruising Range:
   100 miles.
Fuel Consumption:
   0.7 miles per gallon.
Fording Depth:
   40 inches.
Maximum Climbing Grade:
   60%.
Max. Vertical Obstacle:
   28 inches.
Max. Ditch Width:
   6 feet.

3. Armament

Gun:
   M2A1 Dual Automatic Gun (Bofors system).
Caliber:
   40mm.
Weight of Each Barrel:
   271.5 pounds.
Weight of Autoloader and Tray:
   212 pounds.
Overall Weight of Each Gun:
   1,258.5 pounds.
Rifling:
   16 grooves, right twist (twist increases from 1 turn in 45 inches at breech to 1 turn in 30 inches at muzzle).
Max. Rate of Fire:
   240 rpm (both guns on automatic).
Max. Rounds in Rapid Before Cooling Required:
   120.
Muzzle Velocity:
   2,870 fps.
Recoil/Counter-recoil System:
   Recuperator Spring and Oil.
Traverse:
   360 degrees.
Power Traverse Speed:
   40 degrees per second.
Elevation:
   minus 3 degrees to plus 85 degrees in power mode.
Power Elevation Speed:
   25 degrees per second.
Maximum Effective Range:
   1550 meters against aerial targets;
   1850 meters against ground targets.
Maximum Range:
   7,625 yards vertical;
   10,820 yards horizontal.
Barrel Life:
   12,000 rounds.
Ammunition:
   High Explosive Incendiary Tracer
   High Explosive Tracer
   Armor Piercing Tracer
   Target Practice Tracer