With about 140,000 machine guns left over from World War I, the Army felt no urgent need for new production during the 1920's and early 1930's. As time went on, the various wartime types - Lewis, Vickers, Marlin, and others - were one by one declared obsolete, though prudently kept in storage for an emergency, until only the Browning models remained as standard. At Springfield and Rock Island the M1917 Brownings were modified and given new designations, M1917A1 (watercooled) and M1919A4 (air-cooled). The one outstanding machine gun development of these years was the redesign of the .50-caliber Browning machine gun so that it could be quickly converted to serve as tank, aircraft, or antiaircraft weapon. The heavy barrel of the tank gun, the waterjacket barrel of the AA gun, or the lighter parts of the aircraft gun could be attached in a matter of minutes without modification of the basic receiver. This simplified design, adopted in 1933, eased manufacture, maintenance, and troop training throughout the war.

Of the commercial gunmaking firms in the United States, only Colt retained active interest in machine guns during the interwar years. Under contract with Ordnance, it made production studies on Browning guns, both .30-caliber and .50- caliber, and contributed to the preparation of descriptions of manufacture. In 1939 Rock Island installed a production line capable of turning out .30-caliber machine guns at the rate of twenty five per day, and Ordnance placed production orders with the Savage Arms Company of Utica, N.Y., and with four divisions of General Motors - Saginaw Steering, Frigidaire, AC Spark Plug, and Brown-Lipe-Chapin. The British placed contracts with Colt and three other firms to make machine guns for planes and tanks being built in the United States for the British Army, and to all these firms Ordnance released its latest designs. Thus, total machine gun production capacity created during the defense period for the U.S. Army and its allies was considerable. By Pearl Harbor, Ordnance had contracted for annual production of some 430,000 .30-caliber and 300,000 .50-caliber guns, and ten plants, including Rock Island, were in production. supported by scores of subcontractors.

When President Roosevelt announced his "must" program in January 1942, he called for the production of 500,000 machine guns each year for the next two years. The War Munitions Program of February 1942 put total requirements at 1,302,000 for 1942-43 and the first half of 1944. To meet the demand for aircraft guns Ordnance found itself well prepared; it was able to report in February 1942 that output of caliber .50's was running well ahead of plane production, so far ahead, in fact, that caliber .50's were being mounted on 37-mm. AA carriages as additional weapons. General Wesson had told his staff earlier: "Forget everything else, but be sure you have a gun on every plane that comes out of this country; I don't care where it goes, I want a gun for it." For ground machine guns, capacity was below requirements early in 1942 but tank objectives dropped during the year, bringing a corresponding drop in ground machine gun requirements. By the end of the year production of all types, both air and ground, totaled 662,331, just enough to meet requirements. The most sharply defined trend was the shift from the small .30-caliber to the powerful .50- caliber aircraft machine gun with armorpiercing, incendiary ammunition. (Table 16) Hidden within these over-all figures were many stops and starts as requirements were cut and factories shut down at one point only to be followed by an emergency demand for new production a few months later.

The .50-caliber aircraft gun program reached its peak during early 1944 when production capacity rose to 45,000 per month, just enough to meet the Army Supply Program requirement of 540,000 for the year. With 1945 requirements set at 747,000 guns, Ordnance planned to build two additional plants, but dropped the matter during the second half of the year when requirements were cut and surplus machine guns piled up in Field Service warehouses. The contract with the Buffalo Arms Corporation, a high-cost producer in a tight labor area, was terminated early in 1945, and Kelsey-Hayes was put in a standby status.

Production engineers worked miracles in simplifying processes, saving material, and speeding production of machine guns. Rifling broaches more than cut in half the time required for rifling barrels. Pearlitic malleable iron castings, known as Arma steel, not only saved scarce steel alloys but proved superior to the steel or bronze originally used for certain machine gun parts. Substitution of castings, stampings, or a combination of stamped and riveted parts for completely machined parts saved countless man-hours and machine-toolhours, and resulted in lower costs. There were problems, too, that had to be ironed out through the Machine Gun Industry Integration Committee. Some problems stemmed from the fact that commercial machine gun production had been on a small scale during the preceding two decades, and had been limited to one manufacturer. Colt's small peacetime orders had not warranted extensive application of mass-production techniques. Tolerances, specifications, and inspection rules had not been worked out in sufficient detail to guide numerous producers unfamiliar with gunmaking, nor were the rules always applied in the same way. The General Motors plants, for example, complained that inspectors were rejecting parts for exceeding tolerances on the drawings although the parts were identical with those taken from sample guns made by Colt. Tripods were the chief bottleneck in 1941 and could be turned out in adequate quantities only by adopting less complicated designs. An Army inspecting officer reported in April 1943 that, although production to date had been "extremely satisfactory," some of the older plants clung to outmoded methods, resisted change, and were generally less efficient than the newcomers. But, all things considered, machine gun production was one of the most successful features of the whole Ordnance program.

To manufacturers, metallic belt links were deceptively simple in appearance. Each steel link consisted of three small loops, two on one side and one on the other. A belt of ammunition was formed by placing the single loops of one link between the two loops of the other and inserting the cartridge through the three loops in much the same manner as one slides a bolt through the hinge of a screen door. Any number of links could be assembled in this manner to make a long belt of cartridges that had great flexibility and could be rolled and twisted to fit confined spaces in airplanes.

In addition, the belt links automatically fell apart as the cartridges that held them together were fired and ejected from the gun. Thus the origin of the term "disintegrating metallic belt links." Though simple in design these metallic links demanded exceptional accuracy in piercing, cutting, forming, and heat-treating to guarantee faultless performance when used in aerial combat. If links were too hard they were likely to break under pressure, and if too soft they might stretch and cause stoppage of the weapon. If either too loose or too tight they would not function properly. As rust or corrosion on links would render them unfit for use, they had to be given a carefully controlled rustproofing treatment before being sent to the field. Rigid inspection was essential to guard against acceptance of a single link that might cause trouble, for it was literally true that an ammunition belt was only as strong as its weakest link. During the years of peace Rock Island Arsenal was the sole producer of links in the United States. As there was but a trickle of new ammunition produced each year the need for links was correspondingly small, but during 1940 Rock Island turned out about 50,000,000 .30-caliber links and about 15,000,000 .50-caliber. With requirements for aircraft ammunition on the rise, and with a shift toward the larger caliber taking place, Rock Island placed contracts with industry for .50-caliber links, beginning in June 1940 with the Fort Pitt Bedding Co. and three other concerns in 1941. Approximately 150,000,000 were produced in 1941, three times the 1940 output. In the summer of 1941 production of .30-caliber links began at Jackes Evans Manufacturing Company and General Aviation Equipment Company. After Pearl Harbor, requirements for both sizes combined rose to eighteen billion for the 2-year period 1942-43. To meet these astronomical requirements a speedier production process was introduced, using a progressive multi-station die developed at Rock Island in the 1930's, and contracts for link production were placed with many different firms. To speed production and break bottlenecks a Metallic Belt Link Industry Integration Committee was formed in the summer of 1942, and by September 1943 the monthly rate of production had reached more than half a billion. Thereafter requirements were reduced with some contracts being terminated outright and others continued at greatly reduced rates. Early in 1945, after the Battle of the Bulge, there was a brief period of rising requirements followed by contract cancellation as the end of the war appeared in view. Total production of metallic links during the 1940-45 period reached close to thirteen billion.

As far back as 1926 the Secretary of War had tried to hasten the process of standardization of all matériel, but in 1935 the Chief of Staff was still concerned with the problem. At its root lay a philosophy of perfection held by many members of the General Staff, the using arms, and the Ordnance Department itself. General Malin Craig, Chief of Staff from 1935 to 1939, proposed a system of yearly standardization and annual revision that resulted in a directive ordering use in the 1937 program of only standardized equipment, and freezing the design of standardized items from the moment cost estimates were submitted until manufacture was concluded.

But this measure was at best only an alleviation. Difficulties continued. Unequal rates of standardization of closely related items caused great trouble. For example, the 37-mm. infantry gun and carriage was standardized in 1937 but had no ammunition approved for use. Furthermore, unequal rates of standardization of particular components held up acceptance of end items. Of the search for ideal weapons and the delays that search entailed, one observer later wrote: "The best is the enemy of the good."

Not all development projects, to be sure, ran an unduly long course. Modification and redesign of machine guns, begun immediately after World War I, made comparatively rapid progress. Efforts to improve the ballistic and cooling characteristics of the earlier .30-caliber Browning machine guns produced the M1919A4 in 1925, while later collaboration of Ordnance, Air Corps, and Colt Company representatives developed the .30-caliber M2, which could be either fixed or flexible and permitted either right- or left-hand feed. Similarly useful work on mounts went forward.

Still more significant in terms of World War II was the development of the .50-caliber machine gun. In 1930 when the water-cooled .50-caliber Browning machine gun, M1921A, was standardized, the Coast Artillery was satisfied, but Air Corps, Infantry, and Cavalry still lacked what the Chief of Ordnance described as "suitably specialized Brownings." The Air Corps needed lightness, rapid rates of fire, and right- and left-hand belt feeds; tanks required heavy barrel guns with reliable cooling systems.

Neither the using arms nor the Ordnance Department believed it possible to have a single machine gun serve several diverse purposes, but in the two years between 1931 and 1933 Dr. Samuel G. Green of the Ordnance Department succeeded in modifying the Browning to make a single basic gun which, varied by special features for special purposes, could meet requirements for all services. The new model, the .50-caliber M2, was so designed that the operating mechanism was the same for each type of gun. The heavy barrel of the tank gun, the water jacket, sleeve, and 45-inch barrel of the antiaircraft gun, and the lighter parts of the aircraft gun, could each be affixed without modification of the receiver. Here was an outstanding achievement, the benefits of which were to be felt all during World War II; manufacturing, maintenance, and troop training were all eased by this simplification of design.

When the Spanish Civil War provided evidence of the operational value of various items of ordnance, American experts began to question whether the .50-caliber machine gun were not really obsolete both for aircraft and antimechanized use. The using arms therefore ran large-scale tests of the .50-caliber in competition with several types of light automatic cannon. The verdict was in favor of the machine gun.

The problems antiaircraft batteries had to deal with in countering the powerful attacks of Axis aircraft were matched, perhaps more than matched, by those the Army Air Forces encountered. Aircraft manufacturers in the United States during the 1930's had been building planes capable of ever-increasing speeds, but as specifications for military craft had slighted the concomitant developments - faster-firing guns, protecting armor, and self-sealing fuel tanks - these netted relatively scant consideration before 1939. Fuel tanks, as an integral part of the plane, were an Air Corps responsibility. Before 1940 the Air Corps had rejected the idea of armored planes, and only when General Henry H. Arnold insisted, after checking the reports of the Battle of Britain, was any attention given to use of protecting armor. Testing suitable materials and preparing specifications then fell to the Ordnance Department because of its experience in ballistics and knowledge of the behavior of plate on armored vehicles under fire. But the major task of the Ordnance Department in developing matériel for the Army Air Forces was to design guns and ammunition for attack.

When President Roosevelt in January 1939 requested of the Congress vast sums of money to produce 50,000 aircraft, only Ordnance experts and a handful of Air Corps officers fully appreciated the need of equipping these planes with very fast firing guns, with more guns per plane, or with both, in order to score hits on enemy planes traveling at new high speeds. General Arnold, to be sure, as early as the summer of 1937 had requested the Ordnance Department to increase the cyclic rate of the .50-caliber aircraft machine gun, and the Springfield Armory and Colt's Patent Fire Arms Company had spent some time on study of the problem. But funds were skimpy, and progress had been proportionately slight. Furthermore, the Air Corps had submitted no list of required military characteristics. Whether to add more guns to a plane to increase its volume of fire was a matter for the Air Corps to decide; the quality of those guns, in keeping with the user's specifications, was up to the Ordnance Department.

American fighters in 1939 carried nothing heavier than .30-caliber and .50-caliber machine guns in multiple mounts on the wings, fuselage, and nose of the plane. Though the standard .50-caliber Browning machine gun M2 of 1939 could fire 600 rounds a minute on a rigid test mount, in some planes that rate was reduced by about 100 rounds a minute, partly because of the aircraft mount's resilience and partly because of the heavy ammunition-belt loads on the gun's feed mechanism. The .30-caliber aircraft gun fired 1,200 rounds a minute, but its muzzle velocity was low and its bullet light. Aircraft guns had to be mounted in confined spaces, so that any redesign or modification must, if possible, retain the external dimensions of the originals in order to minimize modification of the plane to accommodate the new models. Not until late September 1939 did the Air Corps establish the military characteristics wanted in a machine gun of cyclic rate rapid enough to be effective in the shortened "on-target" time of new planes. By then it was clear that what would suffice to hit a target moving at 200 miles an hour would not serve against aircraft flying at 300 to 400 miles an hour.

Accordingly, the Air Corps' major requirement for the new gun was a cyclic rate of at least 1,000 rounds a minute and as much more as other features would permit. The time of bullet flight was to be .7 second for 600 yards and at that range penetrations of .75-inch armor plate must be possible when armor-piercing bullets were used. The over-all length of the gun should be kept within 68 inches and weight as low as was consistent with efficient performance. Other requirements were full automatic fire, controlled by hand and by trigger motor, right-hand and left-hand ammunition feed, adaptability to mounting for either fixed or flexible use, and the least possible trunnion reaction, that is, the lowest possible strain on the shafts upon which the gun was mounted in the plane. Air-cooling the gun with an aerodynamic barrel jacket extending at least 20 inches aft of the muzzle was listed as essential. The Colt's Patent Fire Arms Company, which owned John Browning's patents, undertook design of a gun to meet these requirements, only to have each model tested during the next two years show such serious defects as to be wholly unsatisfactory. In 1940 engineers at the Springfield Armory, by lightening the barrel of the standard M2 machine gun and by substituting double driving springs for a single spring, pushed the cyclic rate up to 800 rounds a minute, but that was still far below the 1,000 rounds a minute the Air Corps wanted.

Increasing the cyclic rate of a gun such as the .50-caliber M2, designed to fire at 650 to 800 rounds per minute, resulted in added stresses upon the barrel and all the moving parts. Even if other components were strengthened enough to avoid excessive breakage, the problem of barrel erosion would remain. The hot gases generated by the explosion of the powder charge softened the bore surface, the chemical composition of the powder attacked the metal, and the high temperature and pressure tended to expand the bore in a very fast-firing gun. Inaccuracy would therefore soon develop unless barrel erosion could be lessened by cooling the barrel, by using a heavier barrel, or by improving the barrel's metallurgy. In February 1942 the Ordnance Department requested NDRC to study the whole problem. NDRC let contracts to some twenty-six companies, universities, and other research institutions, each of which followed out a particular line of investigation. Two and a half years of work, notably that of the Crane Company and the Geophysics Laboratory of the Carnegie Institution, produced a liner of a special alloy which, fastened into the breech end of the barrel, greatly reduced erosion. Further experimentation showed that combining this material at the breech end with chromium plating extending to the muzzle end gave still higher erosion resistance and better general performance.

In the interim, efforts to develop a machine gun of high cyclic rate had continued. Early in 1942 the High Standard Company of New Haven offered a promising design, though the models tested that summer at Aberdeen lacked both the strength and the reliability to be acceptable. Later models, built under a development contract with the Ordnance Department, showed marked improvements but still failed to meet all requirements. Throughout, High Standard worked on the basis of designing a high-speed gun in which changes from the M2 would be kept to a minimum. This came to be a big handicap. Consequently, in August 1943 the Ordnance Department entered into contract with the Frigidaire Division of General Motors Corporation, with the understanding that a gun be developed using the basic mechanism of the M2 aircraft gun but with no restrictions upon the number of changes that might be made. In short, the Ordnance Department abandoned any plan of having parts of the new gun interchangeable with those of the M2. Frigidaire's first model was ready for test in March 1944. It was essentially a new gun; only minor components were interchangeable with the standard M2. Numerous changes were still needed but, by adopting some features of the High Standard experimental models, Frigidaire succeeded by the fall of 1944 in producing a weapon, the T25E3, that had a cyclic rate of 1,250 or more rounds per minute and functioned well enough to warrant fabrication of a hundred for AAF and Navy service test. By the next April the guns that had been carefully service-tested at Wright Field were proving so far superior to the M2 that the Air Forces requested immediate standardization of the new model. The latter then became the .50-caliber aircraft gun M3, and the M2 was reclassified as limited standard.

Development of a machine gun with cyclic rate increased from 800 to 1,200 rounds per minute, at the cost of only a pound in weight and with no significant change from the over-all dimensions of the slower-firing M2, was so impressive an achievement as to merit some particulars. It was accomplished largely by twelve new features. First was a bolt of improved metallurgy and design with holes drilled through to reduce weight. Second was an extractor with a reversible ejector which eased ammunition feeding. Third was the substitution of a pneumatic barrel buffer for the older oil buffer, a change that produced smooth operation regardless of extremes of temperature. Alteration of the curvature of the accelerator resulted in more effective use of the energy of the barrel and barrel extension to accelerate bolt recoil, while a Belleville spring back-plate buffer, using cupped steel washers, accelerated counterrecoil of the bolt. Rigidly mounted breech-lock depressers added to stability of the gun's components during operation. Redesign of the back-plate and of the breech-lock cam strengthened the construction and gave smoother functioning, and an improved firing pin provided about five times as long a life as that of the M2 firing pin. The two features incorporated from the High Standard guns were a special cover assembly to increase ammunition belt-life capacity and split beltholding pawls to improve ammunition feeding. Use of the new erosion-resistant lining for the barrel, moreover, permitted firing long bursts without loss of accuracy or marked drop in velocity.

Unfortunately, few M3's saw action in World War II, as only some 2,400 were completed before September 1945. Yet in the fall of 1944 Ordnance engineers perceived that some desirable proven features of the High Standard and Frigidaire highspeed models could be readily applied to the M2. Among the parts to be used were High Standard's extractor assembly, recoil booster, wide top-cover assembly and split pawls, as well as two parts designed by Frigidaire for the still unperfected T25E3 model. With improved metallurgy, use of a lined barrel, removal of the oil from the oil buffer, and one or two lesser changes, this modified M2 gun, the T36, could fire slightly over 900 rounds a minute. In October 1944 the Air Forces requested 31,336 of these, but rapid progress on the T25E3 limited output to some 8,000.

Encouraging though these developments were, still faster-firing aircraft guns would clearly be advantageous. The Germans, indeed, believed that extremely high cyclic rates would offer the best possible method of combating high-speed craft. Toward the end of the war observers reported on a German development program to introduce aircraft machine guns with cyclic rates well beyond those under development in the United States. The object was to lay down a dense curtain of fire from a very short range. These experimental guns fired short bursts, with firing initiated photoelectrically when the plane was in the proper position. From the American point of view the great drawback of the device was that it did not permit continuous or prolonged fire, a feature the AAF regarded as more important than a single, very fast burst. In 1945 the maximum rate the Ordnance Department set as its goal was 1,500 rounds a minute. Springfield Armory for a time worked on a design of a totally new mechanism suitable for highspeed operation, and Frigidaire designers continued efforts to increase the rate of the M3 to 1,500 rounds per minute, but the war ended before either type had reached the stage of extended testing.

Improvement in the metallic link belts for aircraft machine guns was also needed to make them sturdier and flexible enough to use in restricted spaces such as the plane's tail. The complexity of the problem may be envisaged by study of the accompanying photograph of an installation. A project to improve the link began in the spring of 1940, and, while the Air Corps accepted a modified link early in 1941, search for a design still more serviceable and easier to manufacture continued throughout the war. If, in addition to getting a high degree of strength combined with great flexibility under strain, a lightweight link could be made, the advantages for aircraft guns would be important. Before V-J Day designers tried some twentyeight variations of the standard one-piece link and several two-piece types. Not only design but also materials and heat-treating affected performance in all metallic links. As the light weight of aluminum and plastic suggested substitution of these materials for steel, considerable study of a nylon link went forward, but the effect of extremes of climate upon plastic links and the inelasticity of aluminum prevented development of anything as satisfactory as the steel types used in World War II. In these, lowering the hardness of the steel largely overcame brittleness. Addition of a sprocket to serve as a booster to the feed mechanism also relieved strains.

Faster-firing guns were not the only answer to the greater speeds of enemy aircraft. Flatter trajectory of fire obtained by higher muzzle velocity was an even surer answer and was sought simultaneously. The NDRC study of hypervelocity noted at the end of the war that, in attacking targets moving in three dimensions, heightening the muzzle velocity of guns by 50 percent would more than triple the number of hits. In any given gun, unless the barrel were lengthened, thereby increasing the weight of the gun, or unless the metallic ammunition components were changed, the only way to increase muzzle velocity without adding to pressures was to use higher-potential powder. But higher-potential powders, like increased cyclic rates, spelled the probability of excessive barrel erosion. This could be avoided only by keeping the temperature of powder combustion low. Extensive experimentation, largely at Frankford Arsenal, produced .50-caliber ammunition with muzzle velocity heightened from 2,700 to 2,880 feet per second, but even with lined barrels the danger of "keyholing" after firing relatively few rounds forbade more powerful charges. As the interior surface of the barrel wore, the projectile tended to deform so that it lost velocity and tumbled in flight, and upon impact made a keyhole-shaped mark. If the barrel became excessively hot the bullet might even break through the wall of the barrel. At the very end of the war tests at the Ordnance research and development center indicated that a newly developed lighter-weight cartridge, the armor-piercing-incendiary T49, using a single-base powder, would give muzzle velocity of over 3,400 feet per second, but the erosion properties of the T49 were still a drawback.

The German point of view on high muzzle velocities in aircraft weapons offers a contrast to the American concept: Investigation of German small arms development and production revealed that, as concerns automatic weapon design, the attainment of a high cyclic rate appeared to be a primary consideration. This was particularly true in the case of automatic weapons designed for aircraft use. Apparently German authorities believed that a relatively low muzzle velocity was acceptable if a high rate of fire could be obtained. High velocities required larger and heavier rounds with a consequent reduction of cyclic rate . . .

Use of power-driven turrets developed by the Air Forces to enable the aircraft gunner to locate his quarry quickly and track him accurately was another means of dealing with the problem of hitting a target moving at high speed. There remained the human factor in marksmanship. Whatever the perfection of gun mechanisms, Army and Army Air Forces both knew that soldiers must be well trained in their use; few weapons were highly effective in the hands of the inept or inexperienced. Where speed of hand and quickness of eye were so vital as to the aircraft gunner, his training became of more than usual importance. Skeet shooting, firing at a towed airborne sleeve or "drone," and shooting a camera gun were used in the early part of the war, but the shortcomings of those methods of training were so obvious that the Air Forces in September 1942 requested a conference with the Ordnance Department to discuss ways of improving upon them. The Air Forces proposed that the Ordnance Department develop a projectile that would disintegrate upon impact without harm to the target or its crew, but a projectile with ballistics similar under training conditions to those of the service ammunition under combat conditions. The Air Forces, agreeing that a suitably armored target plane would be essential to successful use of a frangible cartridge, undertook to develop an armored plane. Two months later the Air Forces changed its mind about the armored target plane and informed the Ordnance Department that unless a bullet could be made that would shatter against an unarmored craft, the whole project must be abandoned. The Ordnance Department dropped it. Research men of NDRC later charged that "one or two willful men in the Ordnance Department nearly stopped the development altogether." The Ordnance staff averred that the Air Forces had tied its hands.

At this point, one or two Air officers, convinced of the value of the idea, persuaded the Air Forces to turn the problem over to NDRC and approve a research contract with Duke University. Experiments were to be confined to work upon a .30-caliber bullet because, though .30-caliber machine guns were ineffective in combat against the armor plate of World War II planes, the firing characteristics were nearly identical with those of the more powerful .50-caliber, and the smaller-caliber ammunition was cheaper. In the course of the next year men at Duke and NRDC came up with a 90-grain bullet of powdered lead bonded with bakelite that, fired from a modified .30-caliber aircraft machine gun, at a distance of 50 yards disintegrated against a quarter inch plate of dural placed normal to the line of fire, or, if the bullet perforated the plate, caused little or no damage. This achievement was encouraging, though it still did not meet the Army Air Forces specifications. The Air Forces then requested the Ordnance Department to continue the development, but Maj. Cameron Fairchild, AAF, Professor Paul Gross and associates at Duke, members of NDRC, and the Bakelite Corporation, who had been the chief proponents of the program thus far, largely saw it through.

The technical difficulties were various. Beside making a frangible bullet with the proper ballistics and adapting the machine gun to firing ammunition with reduced powder charge, a plane had to be built, armored enough to be safe and yet able to fly. A hit indicator system had to be devised and a plan worked out for vectorial scaling of bomber and fighter velocities, for the bullet muzzle velocity, and for the ring sight size. The bullet T44, finally produced in some quantities and used at several training fields toward the end of the war, was slightly heavier than the first experimental type. The machine gun was satisfactorily modified and the scaling problem solved. The Air Forces did build an armored plane upon which hits were scored automatically by electrically amplifying the vibration caused by the bullet's impact and thus flashing a light on the nose of the plane. Yet in spite of these successful developments, only a small fraction of bullets manufactured were fired. After the war Air Forces psychologists concluded that trainees were prone to get false ideas of aiming and firing because the bullet simulated too much. The frangible cartridge was then declared limited standard.

As an aid to accurate fire, tracer ammunition had long been held in high regard by the Air Corps. In 1924 an improved .30-caliber tracer cartridge was standardized as the M1, the combat characteristics of which remained unchanged until Pearl Harbor. But after the Battle of Britain in 1940 the British, though praising tracer as a medium of fire control, wanted a controlled length of trace and, in order to minimize the blinding effect upon the gunner, a delay of 150 yards before the brightburning powder ignited. Satisfactory types were developed in the course of the next two years, but Obsoletion of .30-caliber machine guns for Army aircraft resulted in limiting .30-caliber tracer to ground use and, in small quantities, to use by the naval air forces. Caliber .50 tracer, on the other hand, mounted in importance as the war progressed. A type standardized in 1931 as the M1 formed the basis of the .50-caliber tracer most widely used by the Air Forces during the first two years of the war, though improvements were made in the original bullet by changing tracer and igniter mixtures, by increasing velocity 330 feet per second, and by developing clad-steel bullet jackets. Frankford Arsenal modifications of the M1 produced ammunition with a controlled bright trace of 550 plus or minus 50 yards, and later a type, standardized as the M10, which not only eliminated risk of blinding the gunner and gave a sufficient glow during the first moments of flight to permit him to retain trace image but also had ample intensity to enable him in daylight firing to follow the trace for the duration of the ignition. Thus, the M10 could be used in both night and day combat. It had, moreover, the advantage of longer life with less deterioration in storage than several other types of tracer ammunition.

Another variation of the M1 tracer was developed between 1942 and 1944, a socalled headlight tracer, which gave a frontal visibility three times as bright as the M1. Initial reports from active theatres indicated that the psychological effect upon enemy pilots gave this bright-burning tracer particular value. The Tenth Air Force in December 1944 stated: "Preliminary reports indicated that they (caliber .50 headlight tracer cartridges) make adjustment of fire easier. Enemy pilots seem to be less aggressive and show a tendency to break off combat at longer range than with standard ammunition . . . ." And the Commanding General, Strategic Air Forces in Europe, cabled: "Brilliant tracer indicates enemy fighter to other gunners in formation, which enables our planes to spot enemy aircraft more effectively at greater distances. . . . (This) ammunition is an important factor in breaking up enemy fighter attacks at extreme ranges." Furthermore, pilots at that time believed that this tracer used in ground strafing disturbed flak tower operations. It was therefore standardized as the caliber .50 headlight tracer M21. Only after the war did the Air Forces conclude that enemy pilots had been less easily scared by this ammunition than first reports stated. The M21 was then dropped.

Rapidity of fire, flat trajectory, and accuracy of aim might solve the problem of scoring hits against enemy planes flying at high speeds, but if shots failed to disable the plane or crew, the effect of accuracy was lost. Greater striking power could be achieved by increasing machine gun muzzle velocities, by using air cannon that fired bigger projectiles, or by employing rockets. The threat of encountering enemy armor plate capable of withstanding .50- caliber machine gun fire did not actually materialize in World War II, but bullet penetration of the plane's armor did not necessarily knock out aircraft. Enemy use of self-sealing fuel tanks necessitated development of effective incendiary ammunition.

A .30-caliber- incendiary had been used in World War I but, because of difficulties in manufacture, was later discarded for tracer. When in 1939 and 1940 tests of incendiary characteristics of the tracer showed it to be unsatisfactory, search for suitable incendiary ammunition began again. The Chief of Cavalry and Chief of the Air Corps in July 1940 submitted requirements for incendiary .30-caliber and .50-caliber rounds that upon striking would ignite a gasoline or oil tank or pipelines from the tank. Time of flight was to be approximately that of standard ammunition and the center of impact was to be within twelve inches of the center of impact of standard Air Corps ammunition. Fortunately the British had made some progress with the problem. During the Battle of Britain in the fall of 1940 they had employed a .303-caliber incendiary cartridge that was effective against German bombers. But, like the World War I incendiary, its design was so complex that simplification was essential for quantity manufacture. Frankford Arsenal, assigned the task of redesigning the British .303 both to adapt it to mass production and to convert it to .30-caliber, succeeded in evolving a satisfactory bullet and cartridge by September 1941. This became the .30- caliber incendiary M1 and was issued linked in the ratio of two armor-piercing-two incendiary-one tracer, until in 1943 the Army Air Forces discarded .30-caliber machine guns altogether.

More urgently needed was an effective .50-caliber incendiary. The first acceptable design was the work of the Remington Arms Company, whose staff had already had considerable experience in work on Swiss patents for incendiary ammunition. The Remington development was based upon the British .303 B Mark VI Z and was adopted in September 1941. The bullet was a flat-base type with lead base closure and steel body and was charged with 35 grains of incendiary mixture, 50 percent magnesium alloy and 50 percent barium nitrate. A few months later Frankford produced a type of .50-caliber boattailed bullet that equalled Remington's in performance and proved better adapted to mass production. The Frankford design was standardized and the Remington became the Caliber .50 Incendiary M1 Alternate.

By 1942 flyers had come to regard some type of incendiary as indispensable for air combat. "These pilots, who are in daily conflict with the enemy, swear by the effectiveness of the incendiary ammunition and would as soon go up without their machine guns as without this type of ammunition." But the M1 incendiary did not serve every purpose. In the spring of 1943 the air forces were suffering heavy losses of B-17's in daylight bombing operations over Europe, partly because the M1 incendiary, though excellent against enemy fighters approaching from most angles, was ineffective against frontal attack. The protection afforded by the engine of the enemy craft served to exhaust both the incendiary and the penetration energy of the projectile before it got to the fuel tank. Ordnance small arms ammunition specialists consequently suggested use of the M8 armor-piercing-incendiary developed for antiaircraft defense. The M8, when manufactured in relatively small quantities, proved more efficient than either armor-piercing or standard incendiary rounds, but, when manufactured by mass production methods with the types of powder then available, retention of its high velocity became impossible. Inasmuch as armor-piercing-incendiary with less velocity lost most of its penetrating and its incendiary properties, the Ordnance Department recommended that until something better could be perfected the M1 incendiary continue to be used for general air combat and straight armorpiercing for ground strafing. The something better than either standard incendiaries or the M10 tracer emerged in the spring of 1944 in the T28 armor-piercing incendiary tracer standardized in March 1945 as the M20. Air Forces theatre commanders were authorized to request such quantities as they saw fit.

Meanwhile, in the winter of 1943-44, increasing German employment of jetpropelled aircraft burning kerosene created the need for .50-caliber ammunition capable of igniting aviation kerosene. Half a dozen different Ordnance plants worked on the problem. The Des Moines Ordnance Plant produced the most satisfactory model, a 500-grain bullet containing 90 grains of an incendiary mixture composed of 50 percent magnesium aluminum alloy, 40 percent barium nitrate and 10 percent potassium perchlorate. A single-base powder was used that was found to be superior to double-base powder for firing extended bursts. Quantities of the Des Moines cartridge, listed as the T48, were shipped to the theatres in the winter of 1944-45 and proved so effective that in May 1945 the T48 bullet was standardized as the .50-caliber M23 and the round as incendiary cartridge M23. A report of June 1945 from Headquarters, U.S. Strategic Air Forces in Europe, was enthusiastic: "Most pilots stated that aircraft burst into flames more readily when hit with this type ammunition in contrast to armor-piercing incendiary ammunition. Many enemy aircraft burned after having been hit only two or three times. . . . One pilot destroyed 10 aircraft on a single mission by firing short bursts." This testimony notwithstanding, design of incendiary and armor-piercing incendiary ammunition remained at the end of the war a problem requiring much additional study.

Study of the effects of severe cold on gun steels and oils was part of the mission of the WinterDetachment sent to Camp Shilo in Manitoba in December 1942. All air ordnance tested there performed satisfactorily if lubricated with suitably thin oils. Nevertheless, inasmuch as improved oil buffer assemblies in machine guns would minimize oil leakage at high temperatures and congealing at low, during 1943 Springfield Armory experimented with use of new synthetic packings, new finishes, and design modifications, until the Frigidaire Division of the General Motors Corporation evolved the pneumatic type buffer that used no oil at all. Two helical springs absorbed the recoil and functioned so well in tests in the fall of 1944 that this mechanical buffer was incorporated as a feature of the M3 aircraft machine gun. Not only did the air-and-spring type buffer dispense with the use of oils, the spring action allowed markedly increased rates of fire. Furthermore, in the modified M2 aircraft gun, engineers found it possible to use the oil buffer without oil.

Experience taught the Air Forces that the .50-caliber machine gun was an eminently reliable weapon for the combat conditions of World War IL The 1,453,829 .50-caliber aircraft guns produced testifies to their usefulness. Nevertheless, by V-J Day indications pointed to the probability that more powerful, bigger guns would be employed increasingly as aircraft structures became heavier and stronger and their speeds still greater.

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TM 9-1225, ORDNANCE MAINTENANCE
BROWNING MACHINE GUN. CAL. .50,
M2 ALL TYPES, AND GROUND MOUNTS
October 23rd 1944
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