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

AN/M2 HB .50 Browning Machine Guns

The venerable .50 Browning machine gun has been standard issue with the American armed forces since 1921.  It has appeared in water-cooled anti-aircraft, air-cooled aircraft (flexible and fixed), and air-cooled ground (heavy barrel) configurations.  The AN/M2 gun is the direct descendent of the M1921.  The receiver of the gun is the starting point for the above versions.  In addition, the gun can be setup to feed from either the left or right sides and can be fired manually or electrically.  The AN/M3 is a modification of the AN/M2 which will be described later.  The gun can be setup to fire full automatic or both semi auto and full auto by the addition of some parts.

AN/M2 .50 BMG Water-Cooled

The .50 BMG water-cooled resembles a scaled-up .30 BMG M1917A1.  It uses a light weight barrel surrounded by a barrel jacket filled with a 60/40 mix of water and ethylene glycol.  A packing on the rear of the barrel seals in the water/antifreeze mix at the rear and a packing at the muzzle end of the barrel (muzzle gland) seals in the water/antifreeze mix at the front.  The barrel support in the trunnion block is brass, as is the front of the water jacket, to minimize corrosion.  The water jacket itself is Parkerized (manganese phosphated).  The water jacket is provided with two ports for connection of hoses and a condenser.  As the gun fires continuously, the water reaches a point where it would boil away.  However, the steam is routed through a condenser to cool it down until it becomes water again. The water is recirculated to the water jacket by convection.  Weight of the gun with a full water jacket is 121 pounds. Because the .50 BMG is water-cooled, it can theoretically never overheat (as long as there is water in the jacket).  Water-cooling prolongs the life of the barrel, especially when long, withering bursts of fire are the norm.  As a result, the .50 BMG water-cooled was the primary light anti-aircraft gun until it was replaced by the 20mm Oerlikon in U.S. Naval service beginning in 1942.  It was replaced in U.S. Army service by the AN/M2HB for anti-aircraft and ground tasks (mid-1942, early 1943).  Most AN/M2 .50 BMG water-cooled guns were converted to either aircraft or ground guns as World War 2 progressed. The water-cooled type is obsolete and is likely to be encountered only in museums.

A Browning AN/M2 Water-Cooled machine gun, setup for the anti-aircraft role, is shown in a still from the 1942 movie “Wake Island.” The boxes under the arm of film star William Bendix is the 200-round ammunition box (cover hinged open). The 200-round box carried its ammunition on a reel inside the box and was quite heavy and bulky. It was used extensively in World War 2 and in Korea, but was replaced by the lighter, simpler tray for 100-round ammunition boxes. The coolant hose for the water-glycol mix is just to the right of the inner ammunition box. The cylinder to the right of the coolant hose is a recoil absorber attached to the anti-aircraft mount cradle.

AN/M2 .50 BMG Air-Cooled (Aircraft – Fixed and Flexible)

The .50 BMG air-cooled aircraft gun was THE major machinegun used by all services' fighters and bombers (and motor torpedo boats) during WW2.  Early attempts to replace it with 20mm designs, most notably the Hispano-Suiza HS404, did not work out until after the war ended.  Early HS404 designs became the AN/M1 and AN/M2 20mm aircraft machineguns which were deemed unsuccessful by the users.  After WW2 ended, the product-improved 20mm AN/M3, M24, and Mk12 guns became standard.  As these guns gained ascendancy, the .50 BMG aircraft gun diminished in importance.  Nevertheless, the aircraft gun is still around today because no other cannon-sized gun [.60 (15mm) and larger] can compete with it for simplicity, reliability, and cost-effectiveness.

An AN/M2 .50 BMG (Aircraft) as received from the factory without sights or trigger or spade grips. This particular gun is fitted with a retracting slide assembly for manual cocking. The collar at the rear end of the barrel jacket is called a trunnion adapter and is used to mount the gun (along with the rear lugs) in aircraft. The hole directly behind the trunnion adapter is for the mounting pin of a flexible cradle mount in non-aircraft installations. (Photo: US Army)

The .50 BMG aircraft is fully automatic, has a perforated barrel jacket that extends from the front of the receiver to the muzzle, and can be setup to feed from either left or right sides.  The gun may be trigger fired or fired by a solenoid trigger.  Cyclic rate of the gun is 850 to 950 rounds per minute. The gun as two sets of forward mounting holes: one set in the receiver for mounting on ground mounts and another set provided by a trunnion adapter for mounting in aircraft.  The aircraft gun's barrel is 36 inches long and is Stellite® and chrome lined for durability.  Weight of the basic AN/M2  aircraft gun is 64 pounds (without sights and mount).  Because aircraft guns recoil much harder than ground guns, the oil buffer is filled with PLS oil. The AN/M2 aircraft gun is used as flexible door guns by Army, Navy, and USMC helicopters.  The AN/M2 aircraft fixed gun is used in gun pods on various aircraft.





Left: The M213 is the AN/M2 .50 (Aircraft – Flexible) modified for use on helicopters. The receiver assembly remains the same configuration as the AN/M2HB. A can-shaped recoil damper replaces the barrel support of the ground gun. The heavy barrel of the ground gun is replaced by a light barrel surrounded by a perforated barrel jacket. A screw-in muzzle brake supports the muzzle end of the barrel and reduces recoil loads on the airframe by venting the muzzle blast. The muzzle brake can be installed to vent either vertically or horizontally. If set to the latter position, the muzzle blast can be in the pilot’s or co-pilot’s ear when the gun is mounted in the forward part of the aircraft and fired forward. Current versions of the M213 use a multi-pronged flash suppressor as an alternative to the muzzle brake, depending on the mission. (Photo: US Army)

Left: The newer GAU-16 version of the .50 aircraft machine gun. Note the special recoil-absorbing cradle and the chute for spent brass underneath the gun. This gun has been setup for right-hand feed from the standard 100-round ammunition can. The simple, hinged top cover allows quick reloads as well as securing the box in the cradle during flight. Just visible, to the right of the front sight in the top picture is the link chute that guides expended cartridge links overboard.

AN/M2 .50 BMG Air-Cooled (Heavy Barrel, Ground)

The AN/M2 .50 BMG in its HB, or ground version, is probably the most widely used heavy machine gun on the planet.  It is used on ships' mounts, small combatants' mounts, light armor mounts, heavy armor mounts, soft-skinned vehicle mounts, ground mounts, and anti-aircraft mounts.  The gun can feed from either side, use either a manual or solenoid trigger, and is distinguished by its short, stubby barrel support (barrel jacket).  The gun's cyclic rate is 450 to 550 rounds per minute.  Weight of the gun is 82 pounds.  The gun is 6 feet long and has a barrel 45 inches long that weighs 28 pounds.  The barrel is designed to act as a heat-sink when the gun is fired in automatic mode.  Maximum effective range is 2,000 yards.  The legendary Marine Corps sniper Carlos Hathcock recorded a 2,500 yard kill against the Viet Cong using a sandbagged AN/M2HB and 8 power Unertl telescopic sight.  The big .50 is still the machine gun against which all other heavy machine guns are measured.  Many 20mm (and larger) guns have come and gone.  Each has been touted as a replacement for the .50.  However, the .50 AN/M2HB is still going strong and its 20mm replacements have been replaced.

The Browning AN/M2HB on its M3 tripod mount setup for left-hand feed and ground use. Marine Corps sniper Carlos Hathcock used his 8 power telescopic sight attached to a similar gun to achieve the longest kill of the Viet Nam War. (Photo: Fabrique Nationale - Herstal)

© 2005 Bob Stoner R4