Ordnance Notes -- by Bob Stoner GMCM(SW) Ret

Ordnance Notes -- by Bob Stoner GMCM (SW) Ret.

AN/M3 .50 BMG Air-Cooled (Aircraft)

The AN/M3 aircraft gun is an extensively modified AN/M2 gun. A massive pneumatic buffer at the rear of the receiver and a muzzle booster cap at the end of the barrel jacket identify the gun. This gun was extensively used on late 1940s and early 1950s fighters until supplanted by 20mm machine guns. No sights are furnished on the gun because the sights are considered a part of the aircraft.

The AN/M2 .50 BMG Air-Cooled (Aircraft – Fixed) is the World War 2 gun that was modified after the war to become the AN/M3 .50 BMG. For fixed aircraft installations (wings, noses, or gun pods), the gun is fired by an electrical solenoid trigger (not shown) and cocked by either a pneumatic or hydraulic gun charger instead of the retracting slide assembly (as shown in the photo). (Photo: US Army)

The AN/M2/GAU-16/A.caliber 50 BMG Air-Cooled (Aircraft – Flexible) component parts are shown in this drawing: 1 – back plate buffer assembly; 2 – driving spring assembly; 3 – oil buffer assembly; 4 – barrel extension assembly; 5 – barrel; 6 – bolt assembly; 7 – receiver assembly; and 8 – retracting slide assembly. (Drawing: cosnerfx.com)

Changes to the AN/M3 gun relative to the AN/M2 are: the bolt assembly, oil buffer body, and barrel extension are modified. The oil buffer is not filled with oil. The breech locking cams are riveted to the inside of the receiver on the AN/M3 instead of being attached to the oil buffer body as in the AN/M2. The bolt switch cam and extractor cam are different. The extractor assemblies are different. The top cover has more massive parts (AN/M3) and is not interchangeable with that of the AN/M2 except as a complete unit. The pneumatic buffer on the AN/M3 is designed to absorb the greater recoil forces attendant to the gun's higher cyclic rate. Cyclic rate of the AN/M3 is 1,050 to 1,150 rounds per minute.

The XM14 gun pod as used by helicopters and attack aircraft in Viet Nam. The gun is the AN/M3 .50 BMG (Aircraft – Fixed) is seen here with its front covers removed. Gun pods are self-contained units with only a 28-volt electric connection to the aircraft for firing. A pneumatic connection is used for the recoil buffer. All ammunition is contained within the pod. Note the recoil booster cone on the end of the barrel jacket to increase the rate of fire. The can-shaped device at the rear of the jacket is a recoil damper. Like the AN/M2-series, the AN/M3 can be set-up to feed from either left or right sides. Just visible at the end of the receiver is the massive recoil buffer. (Photo: US Army)

The most famous use of the AN/M3 was by the American F-86 Sabres against the Communist MiG-15s during the Korean War. The massive firepower represented by the six .50s on the Sabres more than compensated for the two 23mm and one 37mm cannons of the MiG-15 (whose guns had been designed to shoot-down bombers).

The AN/M3 gun we put on our Medium SEAL Support Craft (MSSC) at SEA FLOAT/SOLID ANCHOR was recovered by GOLF platoon of SEAL Team ONE. I talked them out of it as a firepower upgrade for the MSSC. I retrofitted a set of spade grips in place of the pneumatic buffer, replaced the AN/M3 buffer with an AN/M2 oil-filled buffer, replaced the M3 bolt assembly with an M2 bolt assembly, and the M3 top cover assembly with an M2 top cover assembly. The crossbred M2/M3 gun worked, but I was never able to work out its teething problems due to lack of range time. I believe part of this had to do with the timing of the gun and the different headspace (8 to 12 barrel clicks instead of 3 to 5 on the AN/M2HB guns). However, the one time that the gun worked flawlessly was the time we got shot at by a Russian RPG-2 (Chicom B-40). The rocket lost a fin upon launch that caused it to tumble in flight. I saw the launch site and splattered it with the M2/M3 hybrid. One burst ate half of the 426-round ammunition box and it was dead-on.

The hybrid AN/M2-M3 gun as installed on the starboard weapons station of the MSSC of Detachment CHARLIE. This gun closely resembles the AN/M2 .50 (Aircraft – Fixed) gun shown in the first photo. The rear recoil buffer has the spade grips and trigger of the AN/M2HB ground gun as well as its sights and retracting slide assembly. Almost all other differences are internal. The ammunition boxes were attached by elastic (bungee) cords. The zigzag cords looped through the grommets attach the ballistic nylon flak curtains to the interior of the well deck. (Photo: Bob Stoner)

Author’s Note: Both AN/M2 and AN/M3 .50 (Aircraft) guns continue to be used by special operations forces. The AN/M2 is now designated the M213 (or GAU-16/A) and the AN/M3 is called the M3M (or GAU-21/A). These modifications are shown in the drawings and photos that follow. [GAU = Gun, airborne, unit –16/A = 16^th model, aircraft -- from USAF Aeronautical and Support Equipment Type Designation System (ASETDS).]

An engineering mock-up of the M59 helicopter .50 machinegun mount with M213 (modified AN/M2 .50 [Aircraft – Flexible]) as designed for the UH-1 helicopter. The bag on the right catches the spent ammunition links. A deflector plate (not shown) is mounted below the gun to guide the spent brass overboard. The gun cradle uses the standard 100-round ammunition box. Later configurations use the flexible ammunition feed chute and a larger ammunition box. The M213 in the picture is equipped with a muzzle brake that vents the blast of the burned propellant up and down to reduce recoil loads on the airframe. A recoil damper (can-shaped object) at the end of the barrel jacket is also used to reduce airframe recoil loads. M213 guns in DESERT SHIELD, Afghanistan, and Operation IRAQI FREEDOM may also be fitted with a prong-type flash suppressor (GAU-16/A). Cyclic rate for the M213 (GAU-16/A) is 850 to 950 rounds per minute. (Photo: US Army)

A drawing of the GAU-16/A aircraft gun showing the recoil-absorbing (soft) mounting, quick change 100-round ammunition box tray, and empty brass and link disposal chute (large tube). The GAU-16/A (and GAU-21/A) are now equipped with the 5-prong flash suppressors. The simple iron sights are shown in the drawing. Some GAU-16/A and GAU-21/A mounts have been modified to use sophisticated night vision or advanced sighting equipment. (Drawing: cosnerfx.com)

The Fabrique Nationale – Herstal M3M is a modification of the AN/M3 .50 (Aircraft). The gun is modified to fire from an open bolt – unlike all other Browning guns that fire from a closed bolt – to assist in cooling and to prevent cook-off of ammunition caused by a round chambered in a hot barrel. The M3M (GAU-21/A) is remanufactured from existing stocks of AN/M3 guns. An advanced sighting system is fitted as well as iron sights. The gun mount is designed to be a recoil-absorbing (or soft) mount. Spent links and brass are guided overboard by the deflector mounted underneath the gun. Power for the advanced sighting system is attached to the gun mount. Cyclic rate of the M3M is 1,050 to 1,150 rounds per minute. Photos show both right and left feed setups and different sighting systems. (Photos: Fabrique Nationale – Herstal, USA)