The Imperial Japanese Navy and Army Air Forces took two different approaches when they decided to equip their aircraft with 20mm autocannons.
The Navy tested some Oerlikon 20mm autocannons, liked it and started local production of two Oerlikon-types, which they called Type 99-1 and Type 99-2. They’re really local copies of the Oerlikon FF and and the larger FFL. The Oerlikons (and therefore both Type 99’s) had a rather low rate of fire. Type 99-1 had low muzzle velocity in addition.
The A6M Zero was equipped with wing-mounted Type 99-1, muzzles flush with the leading edges. Later versions of the fighter were equipped with the longer Type 99-2 with muzzles portruding from the wing edges. Zero pilots generally didn’t like firing the earlier cannons, they were slow-firing and only good at very close range on slow-moving targets. Pilots relied more on their cowl machine guns. Instead of blasting their foes with a few cannon shots, pilots like Saburo Sakai would hang on their target’s tail and pour their two machine guns on it. And sometimes the target would just keep on flying.
The Army got in a bit later and with an approach I found very interesting. They started out with the Ho-103 12.7mm machine gun, a local version of the American Browning 1921 .50 caliber that Americans nickname the “Ma Deuce”, modified to use shorter cartridges to improve rate of fire.
Then they simply scaled up all the parts, loaded it with explosive shells, and got a very decent autocannon indeed, the Ho-5 , with faster muzzle velocity and higher rate of fire than the Navy’s Oerlikon derivatives. Some have called it the best autocannon of WW2, and even some Americans wonder why their own forces didn’t simply upsize their beloved “Ma Deuce” to fire shells. The Ho-5 was only slightly heavier than the 50 cal Browning, and packed a bigger punch per weight.
(The Americans really did consider cannon-arming their single-engine fighters, as can be seen in the P-39 Airacobra. They tried producing Hispano-Suiza cannons but their contractors botched it.)
The Ho-5 went to equip the Ki-44 Shoki, Ki-84 Hayate and Ki-45 Toryu, and generally, the Army pilots fired their cannon more often than their Navy counterparts.
If the Navy’s aircraft cannons were so inferior, why didn’t they switch to Army gear? Well, there was a fierce rivalry between the two arms. They would even design aircraft that shared no common spare parts, and use mutually incompatible ammunition.
And then there was the cartel of retired-Admirals-turned-contractors supplying the Type-99’s, they were really tight with the Navy and elbowed out any competition, let alone allow Army equipment into the Navy system.
The Navy worked around the Type 99-2’s low rate of fire by putting more of them on their planes when possible: the Kawanishi N1K Shiden and Shiden Kai carried four of them, as did the Mitsubishi J2M Raiden. Late-war A6M5 Zeros complemented their two wing cannons with two wing-mounted heavy machine guns. They would have mounted four cannons in the Zero if only there had been room, and the weight of doubled armament compromised the Zero’s chief advantage: agility.
Late war Japanese Army fighters made do with two Ho-5. It was enough.
Zeros equipped with Ho-5 would have remained useful late-war, retaining its agility while delivering a good punch from longer range. When your enemies are faster than you, you need to be able to hit them even as they pull further away and make the kill before they escape. Having a good firing rate helps to hit the target as well.
It was a strange compliment to the Browning Ma Deuce, that an upsized version of it firing shells became a favourite weapon of the Imperial Japanese Army Air Force.
Caliber: 20 mm (0.8 in)
Ammunition: 20 x 94 (84.5 g)
Weight: 35 kg (77 lb)
Rate of fire: 450-600 rounds/min (some sources put it as high as 950 )
Muzzle velocity: 750 m/s (2,460 ft/s)
Navy Type 99:
Caliber: 20 mm
Length: 133 cm (53 in)
Weight: 23 kg (51 lb)
Rate of fire: 520 rounds/min
Muzzle velocity: 600 m/s (1970 ft/s)
Caliber: 20 mm
Length: 189 cm (74 in)
Weight: 34 kg (75 lb)
Rate of fire: 480 rounds/min
Muzzle velocity: 750 m/s (2460 ft/s)