M42 Self-Propelled Anti-Aircraft Gun
Gary's Combat Vehicle Reference Guide

Standard Military Vehicle Characteristic Data Sheets: M42
Alternative DesignationsDuster
Country of OriginUSA
RoleLow-level air defense, ground support
Date Of Introduction1953
Crew4-6 (commander-radio operator, driver, gunner, squad leader, 2 cannoneers)
Combat Weight24.90 tons (22.59 mt)
Ground Pressure9.1 psi (0.64 kg/cm²)
Length, Overall 20.85 ft (6.36 m)
Length, Hull 19.08 ft (5.81 m)
Width, Overall 10.58 ft (3.23 m)
Height, Turret Top 9.41 ft (2.87 m)
Ground Clearance17.3 in (440 mm)
EngineContinental AOS-895-3 500 hp (373 kw) 6-cylinder, air-cooled, supercharged gasoline.
M42A1: AOSI-895-5 with fuel injection.
TransmissionCD 500-3 cross drive
Range100 miles (161 km). M42A1: 120 miles (193 km)
Fuel Capacity140 gal (530 l)
Road Speed45 mph (72 km/h)
Cross Country Speed25 mph (40 km/h)
Swim SpeedN/A
Fording Depth4 ft (1.22 m)
Side Slope30%
Trench Crossing6 ft (1.83 m)
Vertical Wall Climb2.33 ft (0.711 m)
ArmorRolled and cast homogenous steel: 0.3 - 1.25 in (7.62 - 31.75 mm)
Applique ArmorN/A
Explosive Reactive ArmorN/A
Active Protective SystemN/A
NBC Protection SystemN/A
Smoke EquipmentN/A
Type Mount Typical Ammo Load
40mm M2A1 dual automatic gun Turret - M4E1; 7 round hopper manually loaded by 4 round clips 480.
.30 caliber M1919A4 (replaced by M60) light machine gun Turret right front; flexible; belt-feed 1,750.
Fire Control SystemM38 computing sight, M24C reflex sight, speed ring sights
Main Gun StabilizationN/A
Infrared SearchlightAvailable


The M42 Duster (a member of the M41 Walker Bulldog family of armored vehicles) is a full-track-laying, lightly armored, anti-aircraft vehicle designed for use with maneuver forces against very-low to low-altitude subsonic air attack. Because of its rapid rate of fire, the vehicle's 40mm dual automatic gun proved to be effective as a support weapon against ground targets.

The interior of the M42 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. The seats for the driver (front left) and commander-radio operator (front right) are in the driving compartment. The stowage compartment serves as a base for the gun mount and has stowage space for 12 boxes of 40mm ammunition. The engine compartment houses the main engine, auxiliary generator and engine, transmission, and fuel tanks.

The M13 periscope is used by the driver and commander while operating under combat conditions during daylight, and the M19 periscope (a binocular-type, enlarged-view device) is used when the vehicle is being driven under blackout conditions. Infrared light is projected forward from the blackout headlights to illuminate the field of view. The M19 periscope converts the infrared image to a visible image which is viewed through conventional eyepieces.

AD0700: M42A1
The communications system of the M42 consists of a radio set (AN/VRC-7,8,9, or 10), a radio receiving set (AN/GRR-5), an intercommunications set (AN/UIC-1, replaced by AN/VIC-1), and interphones. This equipment is shock-mounted on support shelves in the driving compartment. The AN/VRC is used for intervehicular and command communication, and the AN/GRR-5 provides air defense intelligence.

The welded armorplate gun mount is an open-topped cylinder in the center section of the vehicle. This mount, supported on a ball bearing race ring, can be traversed 360° in either direction by power (9 seconds) or manually (10.30 per crank revolution). Crew positions for the squad leader (left), gunner (right), and two cannoneers (rear) are in the gun mount. The squad leader commands, via the intercommunications set, either from the commander's seat or the gun mount.

The M42 is not suited to airmobile operations primarily because it is not helicopter-transportable. The M42 is suitable for airborne operations, but it is not not air-droppable and must be air-landed.

The M42 replaced the WWII M19 self-propelled anti-aircraft gun in U.S Army service and was replaced by the Vulcan and Chaparral systems.



The M42 defense design is based on a low altitude or dive attack by a 520-knot maneuvering aircraft approaching from any direction. Defense design factors:


The Duster engagement sequence begins when the squad leader (1) designates the target, estimates target speed, direction of flight, and the angle of dive or climb, (2) and sets these values into the M38 computing sight (8). The gunner (3) engages the power drive mechanism and traverses the gun mount and elevates the dual 40mm guns (4) until the target is centered in the reticle of the M24C reflex sight (5). The sight mechanically computes the lead angle (6) based on target speed and flight direction; super-elevation is automatically inserted based on gun elevation. When target tracking is steady, the gunner reports, "ON." Upon receiving the report, "ON," and having determined that the target is within range, the squad leader commands, "FIRE." The gunner commences firing and continues to fire, while the gun is hand-fed by the loaders, until the target is destroyed or the squad leader commands, "CEASE FIRING."

The normal mode of firing is that of POWER CONTROL, using the M38 computing sight. An alternate mode of operation is that of MANUAL. In this latter mode, the mount and dual gun are moved in azimuth and elevation by hand-operated cranks. When operated in the MANUAL mode, speed ring sights (8) are used for fire control.
AD0700: M42 Operation


The Duster can deliver a high volume of accurate direct fire against enemy personnel, lightly armored vehicles, bunkers, observation posts, crew-served weapons, and similar targets. Indirect fire also may be used, although it is less effective and is worthwhile only in a rather narrow range of circumstances.

Limitations of the M42 in the ground support role include its high silhouette, lack of overhead crew protection, and ineffectiveness against heavily armored vehicles. Due to these limitations, the M42 must not be employed in a tank role or as a self-propelled anti-tank weapon.


The M38 computing sight is the M42's primary means of fire control. It consists of a computing box, two speed knobs with two speed scales graduated from 0 to 700 miles per hour on each knob, a target course arrow mounted on top of a bail, a computing box positioning handwheel,and a main support bar for housing a steel band used to transmit lateral motion to the gunner's M24C reflex sight.
FM 44-2, 1956: M38 computing box

The M24C reflex sight has a light window, reticle and mirror, magnifying lens, and coated glass reflector. The M24C is designed to superimpose a reticle pattern in the gunner's line of sight and is used in conjunction with the M38 computing sight during power operation.
FM 44-2, 1956: M24C reflex sight Army Air Defense Digest: M24C reticle

The speed ring sight is used during manual operation if a power failure or local control system malfunction occurs. It has a rear peep element and a series of eight concentric circles as a front element. The eight circles represent the required lead, at a midpoint range of 1,000 yards, for a target traveling 25, 100, 200, 300, 400, 500, 600 or 700 miles per hour. The rear peep element can be adjusted up, down, left, or right.


Main armament is the M2A1 dual 40mm automatic gun (Swedish Bofors L/60). The 40mm gun is a high-velocity, flat-trajectory, clip-fed, automatic-loading weapon capable of firing 240 rounds per minute (120 per barrel). The cyclic functioning of each gun is automatic from the firing of one round to the next.

The 40mm gun may be either fully automatic or semi-automatic. It is air-cooled and, if fired at maximum rate, will over-heat after about 100 rounds are fired. When over-heated, firing must be suspended and the barrel changed, which takes approximately 3 minutes. Gun tube life can vary from 4,000 to 16,000 rounds, depending on the amount of overheating due to continuous firing.

The extreme deterrent range is the tracer burn-out range of projectiles. At this range, the fire is inaccurate but, if delivered with maximum density, may cause enemy aircraft to take evasive action, break formation, and even abandon their mission. Extreme deterrent range for the 40mm gun is 3,500 to 5,500 yards (3,200 to 5,029 m).

The maximum effective range is the maximum distance within which the gun may be expected to fire accurately and inflict casualties or damage. It depends on tracer observation, but in addition, is determined by the type of sighting device, lead tolerance, angle of approach, and the average state of personnel training. The maximum effective range for the 40mm gun is 1,800 yards (1,650 m).

The shortest range at which the gun can track a target depends on target speed and the maximum tracking rate of the gun mount. The M42 can track a 600 mph (966 kph) target at approximately 420 yards (384 m) minimum range.

Other characteristics of the M2A1 gun on M4E1 mount:


M25 Dummy Cartridge

Length: 17.60 in (447 mm)
Weight: 4.75 lb (2.15 kg)
Color Identification: Black with white markings

M81, M81A1 Armor Piercing with Tracer (AP-T)

Used against aerial targets, lightly armored vehicles, concrete shelters, and similar targets. The AP-T round has no fuze.

Length: 17.60 in (447 mm)
Weight: 4.58 lb (2.08 kg)
Color Identification: Black with white markings

Horizontal Range: 9,475 yds (8,664 m)

M91 Target Practice with Tracer (TP-T)

Length: 17.60 in (447 mm)
Weight: 4.72 lb (2.14 kg)
Color Identification: Blue with white markings

MK2 High Explosive with Tracer, Self Destroying (HE-T SD)

Used against aerial targets, personnel, and light materiel. The HE-T shells have a supersensitive fuze which bursts on contact. If no contact is made as the tracer burns out, the relay ignition charge is ignited detonating the bursting charge of the shell.

Length: 17.60 in (447 mm)
Weight: 4.75 lb (2.15 kg)
Projectile Weight: 1.985 lb (0.9 kg)
Color Identification: Olive drab with yellow markings

Muzzle Velocity: 2,870 fps (875 mps)
Vertical Range: 5,100 yds (4,663 m)
Horizontal Range: 5,200 yds (4,755 m)
Tracer Burn Time: 7-12 seconds

Mk2 High Explosive Incendiary with Tracer, Self Destroying (HEI-T SD)

Used against aerial targets, personnel, and light materiel. The HEI-T shells have a supersensitive fuze which bursts on contact. If no contact is made as the tracer burns out, the relay ignition charge is ignited detonating the bursting charge of the shell.

Length: 17.60 in (447 mm)
Weight: 4.75 lb (2.15 kg)
Color Identification: Olive drab with yellow markings

TM 9-1300-251-34&P: M81A1
M81A1 AP-T
TM 9-1300-251-34&P: M25, M91, Mk2
M25, M91, Mk2
TM 9-1300-251-20&P: 40mm clip
Loading 40mm charger clips


M42, T141
1953. "Gun, Twin 40mm, Self-Propelled, M42", "Gun, Self-Propelled, Full Tracked, Twin 40mm, M42". Original production model. Replaced the WWII M19 self-propelled anti-aircraft gun.
Standard Military Vehicle Characteristic Data Sheets: M42
1956. "Gun, Twin 40mm, Self-Propelled, M42A1", "Gun, Self-Propelled, Full Tracked, Twin 40mm, M42A1". A fuel injection system was designed for the M42's engine to improve fuel economy and to increase vehicle range by 20%. This resulted in standardizing the M42A1 with the AOSI-895-5 fuel injection engine and classifying the M42 as limited standard.

Defenselink website: M42
Exercise Brave Shield XIX.
Fort Hood, Texas.
U.S. DoD Photo
1024x680, 244K, JPEG
Defenselink website: M42
Exercise Brave Shield XIX.
Fort Hood, Texas.
U.S. DoD Photo
717x1024, 190K, JPEG
Defenselink website: M42
Duster used for target practice.
Melrose Air Force Range.
U.S. Air Force photo
1024x645, 207K, JPEG