Ordnance Philosophies Differed
By Barrett Tillman
How do you kill an airplane? Essentially there are only three methods: Destroy the airframe, such as severing a wing spar. Destroy the powerplant or systems, rendering the aircraft unable to maneuver or maintain altitude. Kill or incapacitate the crew.
The aircraft weapons of the Battle of Britain were often unsuited to the first method, and could be marginal for the second. Thus, the Battle of Britain was a giant three-dimensional laboratory in 20th century technology.
An unusually good fighter pilot got 15 percent hits on an aerial target, and in one RAF test, less than 10 percent of hits on a twin-engine aircraft inflicted significant damage. Therefore, a high volume of fire was needed for any hope of downing an enemy.
The RAF’s prewar biplane fighters carried anemic armament. The Gloster Gauntlet had two machine guns—no different than the Great War loadout 21 years before—while replacement Gladiators possessed four.
When the Hurricane and Spitfire were designed, their higher speeds were a factor not only in performance, but in perceived trigger time. By 1937 bombers were capable of nearly three times the speed of their Great War counterparts, and presumably the 100mph difference between the new monoplanes and their biplane predecessors still meant greater closure rates on enemy bombers. Consequently, fighter pilots would have less time to track and shoot an aerial target.
The RAF, wanting the highest possible volume of fire, installed eight U.S. Colt Brownings chambered in .303, the standard British machine gun caliber. The American guns (designed by the genius John M. Browning, who died in 1926) were a known quantity, having proven reliable with the U.S. .30-06 cartridge. However, the British projectiles left the muzzle at about 2,400 feet per second, roughly 300 less than the American round.
The British Browning was capable of 1,200 rounds per minute—an eye-watering 9,600rpm from eight guns or 160 rounds in a one-second burst. That seemed an awesome amount of “throw weight” but practical experience showed otherwise. With 300 rounds per gun, a Spitfire pilot only had about 15 seconds firing time. His Hurricane counterpart enjoyed 10 percent more.
RAF Fighter Command got much of the equation wrong leading up to the Battle of Britain. With complex, choreographed maneuvers (“Fighting Area Attacks”) intended to disrupt enemy bomber formations, early Spitfire and Hurricane pilots found too little time to settle down and shoot accurately. Moreover, RAF doctrine called for 400 yard zeros, too far even for eight .303 Brownings. The dispersion was too great for a killing blow most of the time, and eventually many RAF aces threw out The Book and harmonized their guns at 250 yards or less.
In combat the results of eight .303s were disappointing. German bombers crashed or forced down in Britain sometimes showed hundreds of hits with only a few striking anything vital, especially engines. Luftwaffe aircraft had armor plate and often self-sealing fuel tanks, which further reduced the lethality of machine gun ammo. Thus, eight .30 calibers could destroy a Bf 109 but might not kill a twin-engine bomber.
RAF Hispano-Suiza 20mm
The British persisted with testing and developing better .303 ammunition. Eventually, a standard load was one gun firing incendiary, two with Buckingham incendiary/tracers, two with armor-piercing, and three with standard “ball” rounds. The latter were of limited use against aircraft but probably remained in service because of insufficient AP quantities.
The best-known feud about fighter armament occurred at the height of the Battle of Britain: whether to continue with machine guns or begin phasing in cannon. There was insufficient time to convert entirely but some progress was made in that direction.
The RAF had begun evaluating 20mm weapons around 1936. While everyone recognized the greater effectiveness of 20mm versus rifle-caliber weapons, there were bureaucratic and technical problems. British observers were impressed with the French-built Hispano-Suiza gun, but considerable time was lost obtaining manufacturing rights, converting from metric to English measurements, and building tooling and facilities. Beyond those concerns, the “Hissos” were troublesome from the start and required constant improvement. Designed to fire between the cylinders of liquid-cooled engines, the guns were not easily mounted in wings, where they had to be laid on their sides to accommodate the drum magazines.
The Hispano-Suiza proved far less reliable than Germany’s Oerlikon-based designs. Only one squadron received cannon-armed Spitfires, beginning in June 1940. In combat, pilots fumed with frustration, repeatedly returning with malfunctioning weapons. No. 19 Squadron—the first Spitfire unit—pointedly requested a return to conventional armament and got on with the war from September onward.
By the time the RAF largely solved functioning problems, the crisis had passed. Late that year, Spitfire Mk IB models went operational with two cannon and four machine guns, setting the standard for the Mk V and most subsequent wartime variants.
Luftwaffe 20mm Oerlikons
Germany also fielded combat aircraft primarily armed with rifle-caliber machine guns though fighters had potent 20mm cannon as well.
Like everything else, cannon possessed strengths and faults. They hit far harder than machine guns, presumably achieving kills with fewer rounds. But cannons were heavier than machine guns (approaching three times as much), had a slower rate of fire and carried considerably less ammunition. Additionally, 20 millimeters tended to function less reliably than machine guns—a crucial factor throughout the war.
The most common Bf 109 in the Battle was the E-3 with two wing-mounted 20mm MG-FF cannon, a modified Swiss Oerlikon design. It fired three-quarter inch projectiles at 1,970 feet per second, cycling at nearly 550rpm. The 60-round drums feeding the cannon provided less than seven seconds trigger time, but the 20mm projectile was large enough to contain explosives, which could cause lethal damage to single-engine aircraft with relatively few hits. The German Oerlikon cannon was more reliable than the RAF’s Hispano-Suiza.
German Machine Guns
The “Emil” also carried a pair of 7.9mm MG 17s each with 1,000 rounds firing through the propeller arc. Because of the lower cyclic rate of synchronized guns, 109 pilots had as much as a full minute of MG trigger time. However, at the start of the Battle numerous E-1s remained in service, with two MG 17s in the wings with 500rpg, affording 25 seconds’ firing time. Reportedly some replaced the wing MGs with cannon as a field modification.
An unavoidable problem with mixed armament was differing ballistic properties. The .303 and 7.9mm cartridges shot “flatter” than 20mm rounds, requiring a compromise in where the guns were “zeroed” or “harmonized” to intersect line of sight. The 109’s Reflexvisier (“Revi”) sight plane crossed the wing-mounted cannon round’s trajectory at about 150 meters, reaching maximum ordinate (17 inches high) at 300, en route to a 500-meter zero. The 7.9 and 20mm projectiles’ paths merged at around 210 meters range—only about one foot above line of sight—and that was where Messerschmitt pilots were most effective.
German ordnance engineers computed that the MG FF-M in the 109E-4 provided nearly twice the destructive potential of the RAF battery. The M (for Minen) was a thin-walled projectile containing more high explosives than previous 20mm rounds, but was not interchangeable with other Bf 109s. However, the HE charge could be far more effective in destroying airframe components than rifle-caliber ammunition.
The heaviest armament in the Battle belonged to the Bf 110C Zerstorer with two FF 20mm cannon containing 180 rounds per gun and four 7.9mm MG 17s, each with 1,000 rounds, plus a rear-firing machine gun for the observer. The 110 could destroy any single- or twin-engine opponent it engaged, assuming the Zerstorer got into position for a decent shot. Because of its potent armament the 110 became the Reich’s primary night fighter after the Battle.
All German fighters possessed fuselage-mounted guns or cannon, from the Bf 109 through the jet-propelled Me 262.
The main advantage of nose-mounted weapons was small dispersion to the limit of the cartridge’s effectiveness. The “garden hose” effect not only produced maximum concentration of firepower on target, but avoided the tendency of wing-mounted guns to malfunction while maneuvering under G load. The philosophy peaked in the Bf 109F which dispensed with wing guns entirely. For the full article, see the August 2015 issue of Flight Journal.