Booster Design on Air-Cooled Ground .30 Caliber Browning Machine Guns
By Charles Brown

When John Moses Browning began work on converting the recoil operated Model of 1917 water cooled machine gun into the air cooled Model of 1919 Tank Machine Gun he faced a couple of problems. He had a proven functioning recoil operated mechanism, but it was designed around a 3 pound, 24-inch barrel. The 18-inch tank gun barrel weighed about 5.5 lb. Due to its increased mass, necessary for heat dissipation caused by the loss of the water cooling feature and since the recoil energy from the M1906 service cartridge remained unchanged, the additional energy needed to ensure reliable operation had to come from somewhere. Browning decided to design a chamber in the front barrel bearing with a restricted orifice to use propellant gasses to exert rearward force on the end of the barrel to supply the extra recoil energy required. The Ordnance Department called these parts the muzzle attachment and muzzle attachment plug. Today they are commonly known as the booster and booster plug.

It is doubtful that any of the tank guns made it overseas as the drawings of the muzzle attachment and plug are dated November 10, 1918, the day before the Armistice ending combat was signed.

Due to the Ordnance Department’s changing the drawing naming convention, these parts for the tank gun took on a new drawing number in 1931 and again with the 1936 approval of production drawings for the M1919A2 and again in 1939 with a change in nomenclature and changes in design for the M1919A4. After 1931 these parts were formally known as front barrel bearing and the front barrel bearing plug.

Since the tank guns were never subject to combat, which is the ultimate test for weapons, extensive testing of the M1919A2 and M1919A4 at Rock Island Arsenal (RIA) pointed out one of several design deficiencies: the barrel bearing and plug had a tendency to vibrate loose. Revision 2 in May 1939 added the cuts and staking points for the sheet metal bands, commonly referred to as lock rings that keep the booster parts attached to the gun. The plugs are also of some interest because of several changes in the orifice diameter. The original tank gun plug used a .872 orifice for the service round of the period, the M1906 Ball cartridge. The army, after about 8 years of testing, adopted the more powerful M1 Ball round in early 1926 as a result of WWI complaints about the range of the M1906 Ball cartridge when used in the Model of 1917 Browning. Not much of this new ammunition actually came into use because of the approximately 2 billion rounds of WWI era M1906 Ball in the supply pipeline.

As the 1919A4 design went through a period of testing and evaluation several changes to the booster were considered. Comments gleaned from Ordnance Committee meeting minutes showed some reservations about mounting a .30 caliber automatic weapon on a lightweight tripod, the 15-pound M2. To alleviate these concerns several types of “stabilizers” were developed.

The stabilizer was a device that combined the features of the gas assist booster with a recoil check/ muzzle brake. Not much came of this as the devices tended to accumulate carbon deposits and proved unnecessary as the M2 tripod proved to be a stable platform for the M1919A4.

1938 brought about another experiment that affected the booster orifice. This time it was an attempt to increase the firing rate of the M1919A4 from 425-450 rounds per minute to 500-600 rounds per minute. This was supposed to be accomplished by making the orifice .617 inches in diameter, which increased the pressure on the end of the barrel and installing a stiffer drive spring. Test firing showed that this combination of parts raised the firing rate to 500 rounds per minute, which was not thought to be of much value when compared to the possible damage to the weapon.

During the run-up to the adoption of the M1919A4 equipped with a 7.5-pound 24-inch barrel the Ordnance engineers upped the plug’s orifice size to .920 and marked the plug for M1 Ball cartridges.

The Ordnance Department promptly jerked the rug out from under this design by re-starting production of the M1906 cartridge in 1936 for use in weapons other than aircraft and belted ammunition. 1940 saw the adoption of what was essentially the M1906 Ball cartridge now known as the M2 Ball as the Ordnance Standard for all weapons other than ammunition contracted for by the Navy.

The Army Air Corps had decided to use only .50 caliber weapons on aircraft but the Navy still used .30 caliber machine guns in the rear seats of bombing and torpedo planes. This flip-flopping resulted in army ground forces using an inferior cartridge design, at least from a machine gun standpoint. The M1 Ball cartridge used a 173 grain boat tail projectile giving it an effective range of about 3,000 yards versus the M2 152 grain flat base with a 2,000 yard range.

The ballistics of the M1 Ball were the rough equivalent of the era’s .30 caliber match ammunition. Many reasons for the adoption of the M2 Ball service cartridge have been floated through the years. The most commonly accepted ones are concerns about the M1 cartridge exceeding the safety zone around firing ranges, causing jams or other malfunctions in the new M1 gas operated rifle, and logistics issues caused by having two kinds of .30 caliber Ball ammo.

In any event it was back to the drawing board for the engineers at RIA. After further testing with the M2 Ball cartridge it was decided to go with a .718 inch orifice, which remained the standard for M1919 boosters, except for the M1919A6, until the end of service life.

The bearings with plugs are commonly referred to as “two piece boosters” and were issued throughout WWII. In July 1942 a new style bearing B221301 appeared. It was a one piece unit lacking the separate plug and lock ring and with a pronounced taper. For reasons not entirely clear, this bearing is commonly referred to as the “A5” booster even though the drawing and Standard Nomenclature Lists specified that it was to be used on the A4’s and the A5. The reason for the taper is thought to be to make it easier to insert the weapon in a combination or ball mount in a tank. Whether or not this is factual is not known.

Throughout WWII the Ordnance Department was constantly trying every possible approach to material conservation and reducing machine time. The B221301 design booster eliminated machining the threads in the bearing body for the plug, the entire plug and the front locking band.

March, 1943 brought an alternate design for the tapered one piece bearing B221301A. This likely took place to again reduce machine time as this bearing lacked the exterior taper and used a different interior design that was easier to machine. This alternate design became the preferred style and replaced the tapered “A5” style completely by February 1945 when Ordnance switched the part design drawing number and the alternate part became the standard part.

The original design of the M1919A6 lacked a booster feature because the barrel contour was changed, which lightened the barrel and by association the recoiling parts enough, it was thought, to eliminate the need for the gas assist boost. This was part of an effort to obtain a front barrel change feature for the M1919s; something that Ordnance had been working on since the mid 1930s. Everything seemed to work fine in testing, but when the weapon was combat deployed in the fall of 1943 in the Salerno campaign and later up the west coast of Italy at Anzio things hit a snag. Complaints from the front regarding a lack of reserve power when the muzzle of the gun was elevated focused attention directly at the lack of the gas assist boost feature.

By now the path back to the drawing board was getting pretty well traveled. Ordnance engineers at RIA came up with a solution. Shorten the A6 barrel jacket, redesign the front barrel bearing to accept a cap style booster with a .820 orifice to accommodate the A6’s lighter barrel and design a clip to hold the cap to the bearing. The front barrel change feature was retained and this new configuration improved weapon reliability. The new parts were available by late spring 1944.

The cap style A6 booster remained in service through the Korean War even though its function was replaced by the M7 booster/flash hider designed in 1945 and produced in quantity by RIA starting about 1949 along with a better retaining clip. The M7 was designed to be a drop on fit to the A6 front barrel bearing.

The original A4 two piece booster had one advantage over the one piece; it was much easier to scrape carbon deposits out of because it could be disassembled. There were two basic hand tools developed to remove the front barrel bearing and the plug or the one piece style booster. The first was the C68334 M6 combination wrench and the B147277 socket wrench. The M6 was a multi-purpose tool used on several weapons, and the oblong hole on one end fit the wrench flats on the end of the M1919A6 barrel. The bearings have two notches and the plug, one piece bearing and the A6 cap style booster have a cut machined into the face that the edge of the M6 wrench fits into. The socket has a bar on the inside that serves the same purpose as the edge of the wrench and the end has wrench flats, and a hole to insert a rod or screwdriver. The oblong hole on the end of the M6 fits over the socket end. The one piece bearing could be cleaned using a carbon removing tool that appears to have been used in mobile repair shops or ordnance depots.

The Israeli Defense Forces received thousands of M1919A4s in the late 1950s as military assistance items, which they promptly converted to 7.62x51 NATO. Since this was a slightly less powerful cartridge the IDF bored out the booster or plug orifice and sleeved it down to 13.5 mm or about .532 inches in diameter.

When the U.S. Navy decided to convert M1919A4s to MK 21 MOD 0 weapons to arm river patrol craft in Viet Nam using 7.62x51 NATO ammunition loaded in M13 links, RIA produced a suitable front bearing with a .525 orifice.

The M37 Brownings used several designs of front barrel bearings some having the booster chamber chrome plated making them easier to clean. Post Korean War, most of the USGI M1919A4s had their boosters swapped out for M6 flash hiders which were combination front barrel bearings and boosters with the flash hider feature. The M1919A6s got the M7 flash hider/booster and the M37 Browning remained the only weapon using the front barrel bearing/booster. As the M1919 family settled into retirement, replaced by the M60GP machine gun, the whole subject of boosters faded into obscurity.

This article first appeared in Small Arms Review SAW (July 2012)
and was posted online on May 18, 2012


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