Japanese aircraft armament was characterised by a wide variety of different types most of which used their own unique ammunition. This is partly explained by the fact that the air forces were divided between the army and navy, although a similar situation did not prevent a high degree of commonality in US aircraft guns. Certainly the Imperial Japanese Army and Navy (IJA and UN respectively) followed wilfully different procurement policies and seemed unconcerned about the benefits of standardisation. In the following, the names in brackets are the Allied code-names for the aircraft.
As with other air forces during the 1930s there was a great reliance on RCMGs, most of which were copies of old foreign designs. By the time Japan entered the war, these had been supplemented by HMGs and 20mm cannon. Later still, larger calibres were being developed under the pressure of attack by USAAF bombers; and towards the end of the war they had B-29 Superfortresses to deal with, which were even more of a challenge than the B-17s normally used in Europe. In this respect, the development of
Japanese air armament mirrored that of the Germans. There were differences, however: the relative unimportance of armoured warfare meant that airborne anti-tank guns were never a priority, and as we shall see there was even more interest in upward-firing guns.
Most of the weapons were based upon foreign designs, mainly Oerlikon and Browning (in some cases with improvements on the originals), but there was one talented Japanese engineer, Dr Masaya Kawamura of Japan Special Steel, who designed some very effective weapons.
The UN adopted two weapons in the HMG class. The 13mm Type 2, used in flexible mountings for the defence of bombers and reconnaissance aircraft, was a copy of the German MG 131 except with percussion rather than electric priming for its
13 x 64B cartridges. Later on, the much more powerful 13mm Type 3 (a version of the Browning M2 slightly modified to fire the Hotchkiss 13.2x99 ammunition already in naval service) was introduced as a fixed weapon in the Kawanishi Shiden (George) fighter from the N1K3-J version and the Mitsubishi Reisen (Zeke) fighter - better known as the Zero - from the A6M5 onwards. A flexible version of the 13mm Type 3 (senkai kikanho instead of kotei kikanho) was adopted as a replacement for the 13mm Type 2, with which the UN apparently experienced problems. A scaled-down Oerlikon firing a
14 x 101RB cartridge was developed but saw no service.
It appears that the IJN was particularly keen on Oerlikon designs. In the 20mm cannon class, the navy adopted (in 1939) and subsequently developed the Oerlikon FF and FFL guns, and was the only major user of the latter weapon. These were known as the Type 99-1 (20 x 72RB) and Type 99-2 (20 x 101 RB) respectively, and differed considerably in length and power. The Type 99-1 bore a close resemblance to the immediately related Luftwaffe MG-FF, including the use of a barrel casing in the flexibly mounted version, although in this instance the casing was perforated to aid cooling and the gun was mounted upside-down so that the drum was underneath. Various versions of each weapon were made for both flexible and fixed mounting, all with drum feed except for the Model 4 (fixed) versions of both the Type 99-1 and 99-2, which were adapted by Kawamura for self-powered belt feed (something the Germans failed to achieve with the MG-FF).
The Type 99-2 replaced the smaller gun in many applications as the need for higher muzzle velocity became evident (and aircraft became powerful enough to carry the extra weight), but the 99-1 remained in service throughout the war, particularly in defensive and upward-firing installations. Although information is incomplete, it appears that a few examples of a thoroughly revised version, the Type 99-2 Model 5, were produced by the end of the war. This had a bolt assembly lightened as much as possible which, together with additional buffer springs at the rear of the gun to accelerate the bolts return stroke, enabled a rate of fire of between 620 and 750 rpm to be achieved. This was a much better performance than either Oerlikon or Ikaria managed with this type of gun.
The Type 99-1 was fitted as a defensive weapon in some versions of the Kawanishi H6K (Mavis) and H8K (Emily) flying boats (in turrets) and the Mitsubishi G3M (Nell) and G4M (Betty) bombers, but was mainly used as a fixed gun in IJN fighters such as the NIK Kyofu (Rex) floatplane fighter, early versions of the A6M (Zeke), the Nakajima JIN (Irving) and C6N Saiun (Myrt) reconnaissance/night-fighter and the Yokosuka P1Y Ginga (Frances) bomber/night-fighter. The Type 99-2 was fitted to the Kawanishi N1K1-J Shiden (George) fighter, the Aichi E16A (Paul) and B7A (Grace), later versions of the Mitsubishi A6M (Zeke) and G4M (Betty), the J2M Raiden (Jack) fighter, and also late versions of the JIN (Irving) and the Yokosuka D4Y Suisei (Judy) dive bomber/reconnaissance/night-fighter.
In larger calibres, the navy developed the 30mm Type 2, a scaled-up version of the Type 99 using a 30 x 92RB cartridge, which was tried in the A6M3 (Zeke), J2M3/4 (Jack), PI Yl-S (Frances) and possibly the J1N1-S (Irving). It was not considered satisfactory, at least in part because of the limited (forty-two-round) magazine capacity, and saw little use. To replace it the navy adopted the Kawamura-designed Type 5, which used a new 30 x 122 cartridge giving a substantial increase in power over the Type 2. Unlike the earlier gun, it was not an API blowback weapon but used a combined gas
and recoil action. It was fitted to one night-fighter versions saw the twin RCMGs replaced by a single version of the C6N1 (Myrt) (mounted in the rear cockpit to fire obliquely upwards), the P1Y2 (Frances) and the J2M5 (Jack). It was planned to become the navys standard fighter weapon but appeared too late.
13mm Type 3; later still two Type 3s were added to the wings, outboard of the cannon. By now, the armament of three HMGs and two cannon was comparable with that of the USN fighters they faced, and probably more effective against
Typical armament installations of UN fighters bombers. Pressed into service as night-fighters, showed a steady increase in power. To give one some A6M5d-S had an additional 20mm Type 99-1
example, the A6M2 (Zeke) commenced the war cannon, with a 100-round drum magazine, installed with two synchronised RCMGs and two 20mm behind the cockpit, firing obliquely upwards.
If the UN was partial to Oerlikons, the Japanese engaged in the Battle of Britain more than a year Army Air Force was equally addicted to the before. Initially, the pilots preferred to use just the Browning short-recoil mechanism, which was
RCMGs as they were easier to score hits with than scaled up or down to use a wide variety of car-
the low-velocity cannon, but they had to learn to tridges from 12.7mm to 37mm. Their heavy use the cannon as increasing US aircraft protection made the RCMGs less effective. The succeeding weapons started (in calibre terms) with their standard HMG - the Ho-103 or 12.7mm Type 1 A6M3 saw the introduction of the more powerful (12.7 x 81SR), a scaled-down Browning M2 which Type 99-2 cannon, which in the A6M5 was was fitted to a wide range of aircraft in both fixed fitted with belt feed (the Model 4) to increase the and flexible mountings.
ammunition capacity to 125 rpg. Later A6M
Among the few weapons which did not use a ammunition capacity to 125 rpg. Later A6M
Among the few weapons which did not use a
Browning mechanism were the first of the 20mm cannon in regular service, the Kawamura-designed Ho-1 and Ho-3. These were modifications of the Type 97 gas/blowback anti-tank rifle and used the same 20 x 125 round. The Ho-1 was designed for flexible mounting and was fitted with a fifteen-round double-drum magazine, while the Ho-3 was used in fixed installations with the fifty-round magazine. Both were powerful but slow-firing weapons. The Ho-1 was carried by the Nakajima Ki-49 Donryu (Helen) bomber in a dorsal turret, while the Ho-3 was fitted to a version of the Kawasaki Ki-45 Toryu (Nick) heavy fighter.
Before the war some use was made of a Type 94 flexible gun' (20 x 99RB) in the Ki-20 bomber. This was reportedly a modified version of the Oerlikon L, the predecessor of the FFL, but it does not appear to have been a success as no more was heard of it. The IJA later imported some 400 MG 151/20, only fitted to some Kawasaki Ki-61-I Hien (Tony) fighters, as a stop-gap until the development of the Ho-5.
The Ho-5 was yet another version of the Browning scaled up to take a 20 x 94 cartridge which was in effect a lengthened MG151/20 case (or a shortened Hispano!). The gun weighed only 37-45kg (depending on type) and achieved 850 rpm which made it, on paper at least, one of the best of the wartime cannon. Although it was adopted in 1942, development continued throughout the war and it suffered from poor quality towards the end of the conflict. The ammunition had to be steadily downloaded to avoid overstressing the mechanism, so it never achieved its performance potential. The gun equipped later versions of the Nakajima Ki-43 Hayabusa (Oscar) fighter and the Kawasaki Ki-45 (Nick) and Ki-61 (Tony) as well as the Nakajima Ki-84 Hayate (Frank) fighter, the Kawasaki Ki-100 fighter and Ki-102 (Randy) heavy fighter, the Mitsubishi Ki-46 (Dinah) - a reconnaissance aircraft converted to a bomber interceptor - and (as defensive armament) the Ki-67 Hiryu (Peggy) bomber.
After initial experiments with a Browning enlarged to take a 25 x 115 cartridge, the IJA decided that more power was needed and developed this into the 30mm Ho-155 (sometimes erroneously referred to as the Ho-105 or Ho-151), which used a
30 x 114 cartridge (case lengths of between 113mm and 115mm have been reported). Two versions of this gun were made, the Model II being a lightened version of the Model I. As with the Ho-5, the later version had a reduced performance to preserve reliability. Like the IJNs Type 5, it was intended to be a major weapon but development was still continuing at the end of the war and it only saw action in the Ki-61-I (Tony) and Ki-84-lc (Frank) fighters.
The IJA also used larger-calibre weapons in air fighting. The early 37mm Type 98, a version of the Type 94 tank gun using the same 37 x 132R ammunition, was manually loaded. The long-recoil Ho-203 (37 x 112R), yet another design by the prolific Dr Kawamura, fired automatically (if rather slowly) and was fed by a fifteen-round magazine. Initially, the cartridge was loaded with a 530g shell and only achieved a muzzle velocity of 430 m/s, but this was felt to be too low so the shell weight was reduced to 475g, permitting an increase to 570 m/s. The Ho-203 was fitted to later versions of the Kawasaki Ki-45 (Nick) and possibly other heavy fighters. The later Ho-204 used a new 37 x 144 rimless case which boosted muzzle velocity to 710 m/s, and the rate of fire also increased to 300-400 rpm by virtue of the usual Browning short-recoil action. It only reached service towards the end of the war in the Ki-46-III (Dinah), in an upward-firing installation, but was tested in other aircraft.
The IJA also used a 57mm aircraft gun, the Ho-401, a scaled-up version of the Ho-203 by the same designer. This was built around a 57 x 121R cartridge, derived from a tank gun round, and was fitted to the Ki-102b (Randy) assault plane and the prototype Ki-93-la heavy fighter. One even larger-calibre cannon also saw limited service: the manually loaded 75mm Type 88, which was fitted to the Mitsubishi Ki-109, a heavy fighter based on the Ki-67 (Peggy) bomber, and the prototype Ki-93-lb.
As with UN aircraft, Japanese Army fighters steadily increased in hitting power. The Nakajima Ki-43 (Oscar), just entering service at the start of the war, was initially still armed with only two RCMGs. Later these were replaced by two 12.7mm Ho-103 and later still two 20mm Ho-5, still a light armament. The vee-engined Kawasaki Ki-61 (Tony), which had four Ho-103 in early versions, also replaced these with Ho-5 later on, with a few
WUAl»«i\S FOll AIR FIGHTING
fitted with two Ho-103 and two of the powerful 30mm Ho-155. It appears that the Japanese were never interested in engine-mounted cannon, even though the Ki-61s engine was based on the DB601 and could therefore have been fitted with one.
Some accounts of upgunning have been exaggerated: the Nakajima Ki-44 (Tojo) which started with two RCMGs and two Ho-103 before graduating to four Ho-103, is claimed by some sources to have carried in later versions 20mm Ho-3 and Ho-5 cannon and even the massive 37mm Ho-203, but primary sources contradict this. However, a small number of Ki-44-II special versions (tokubeti sobi) did carry two of the remarkable 40mm Ho-301 in the wings. The rate of fire of the caseless ammunition was an impressive 450 rpm and the gun weight a reasonable 132kg, but the muzzle velocity was a very low 230 m/s and the ten-round magazine gave a firing time of just 1.3 seconds. These characteristics meant that the weapon could only be used for one attack, pressed home at a suicidally short range. It was not a success.
One unexplained aspect of Japanese aircraft ordnance was the difference in performance between the naval and army weapons. The UN guns were generally quite powerful for their cartridge size, but the IJA guns noticeably less so, particularly towards the end of the war. While the IJAs Browning-pat-tern guns were evidently susceptible to the gradual reduction in manufacturing quality, forcing a reduction in ammunition power, it appears that the UN's simpler Oerlikon-derived weapons were less affected by this problem.
It is worth noting that the Japanese took an early interest in upward-firing gun mountings for night-fighters and put these into service in 1942/3, in parallel with the German Sclmige Musik system. The later prevalence of this type of mounting probably had much to do with the height and speed performance of the B-29, which many Japanese lighters had difficulty in intercepting.
Unlike German practice, aircraft so equipped included single-engined fighters such as versions of the IJNs A6M (Zeke) - one 20mm Type 99-1; C6N
Japanese 40mm Ho-301 ease/ess. Note projectile standing on magazine (Courtesy: MoD Pattern Room)
(Myrt) - two Type 99-2; or one 30mm Type 5, D4Y (Judy) - one Type 99-2; and J2M (Jack) - two Type
99-1, as well as the IJAs Ki-84 (Frank) - three 20mm Ho-5. Twin-engined naval aircraft with upward-firing guns included the J1N1 (Irving) -two Type 99, and P1Y1 (Frances) - four Type 99. The 1JA aircraft were the Ki-45 (Nick) - two 20mm Ho-5; Ki-46 (Dinah) - one 37mm Ho-204; and Ki-102 (Randy) - two Ho-5. The J1N1 -S (Irving) actually had four obliquely mounted 20mm cannon in the fuselage - two firing upwards, two downwards.
Complete information about which models of Type 99 were used in upward-firing installations is not available. The most common weapon was probably the Type 99-1 Model 3, which was fitted with a
100-round drum, but the belt-fed Model 4 and the Type 99-2 were used where there was space for them.
Perhaps the most remarkable armament fit belonged to the twin-engined Kawasaki Ki-45-Kai-C (Nick), which reportedly mounted one forward-
firing 37mm Ho-203 (with a fifteen-round belt feed) in the nose, a forward-firing Ho-3 in a ventral tunnel, two upward-firing Ho-5 inside the cockpit, and an RCMG for the observer! Other reports state that the Ho-3 was in practice removed when the Ho-5s were fitted.
The UN also experimented with obliquely mounted guns for ground attack. The P1Y (Frances) was fitted with up to seventeen downward-firing Type 99 guns in the bomb bay, twelve angled forwards, five rearwards. Thirty of these aircraft were being prepared for attacking B-29 bases and for sweeping landing craft during amphibious assaults, but the war ended before they could see service.
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