Here are the two published sources in English. They happen to agree with each other, even though Dyer adds some more detail:
Ki-93
Rene J. Francillon, Japanese Aircraft of the Pacific War, 1987 Putnam Aeronautical Books, reprinted 1988 -- ISBN 0-87021-313-X -- p.244-245
-- "The Ki-93 was the last heavy fighter and ground attack aircraft built in Japan during the war and the only design of the Rikugun Kokugijutsu Kenkyujo (Army Aerotechnical Research Institute) to be flown. Powered by two 2,400 hp Mitsubishi Ha-214 eighteen-cylinder air-cooled radials driving six-blade propellers, the Ki-93 was designed specifically to carry large caliber cannon in its under-fuselage gondola. From its inception the aircraft was designed to perform as a high-altitude heavy bomber destroyer as well as a low-altitude anti-shipping aircraft. In its bomber destroyer form (Ki-93-Ia) the aircraft carried offensive armament comprising one 57 mm Ho-401 cannon and two 20 mm Ho-5 cannon, while for the anti-shipping missions (Ki-93-Ib) its offensive armament comprised one 75 mm Type 88 cannon and two 250 kg (551 lb) bombs. In both instances defensive armament consisted of a single hand-held 12.7mm Type 1 machine gun. The cockpit and engine nacelles were heavily armoured, and all fuel tanks were self-sealing and had an automatic fire-extinguishing system."
-- "Production of the Ki-93 was entrusted to the Dai-Ichi Rikugun Kokusho (First Army Air Arsenal) at Tachikawa, and the first prototype, fitted with the armament intended for the Ki-93-Ia, was completed and flown in April 1945. The chaotic conditions prevailing in Japan at this late stage of the war delayed the flight trial programme and tests were not completed prior to the Japanese surrender. A second prototype, in Ki-93-Ib ground attack configuration, was completed but not flown before the final collapse."
Armament
-- One 57mm Ho-401 cannon and two 20mm Ho-5 cannon in a ventral gondola and one flexible rear-firing 12.7mm Type 1 machine gun (Ki-93-Ia)
-- One forward-firing 75mm Type 88 cannon and one flexible rear-firing 12.7mm Type 1 machine gun (Ki-93-Ib)
Edwin M Dyer, Japanese Secret Projects - Experimental Aircraft of the IJA and IJN, 1939-1945 (Book 1), 2009 Midland, reprinted 2010 -- ISBN 978-1857803-174 -- p.38-39
-- "When Rikugun Kokugijutsu Kenkyujo began the design research for the Ki-93, the goal was to provide an aircraft that could provide a platform for anti-bomber operations and anti-shipping missions. In both cases the aircraft had to be able to absorb damage when flying in the face of interceptors, the defensive machine guns of bombers, and the anti-aircraft weapons of ships."
-- "Two versions of the all-metal Ki-93 were to be constructed. The first, the Ki-93-I Ko, was the heavy fighter that would combat bombers. The second was the Ki-93-I Otsu and this was the anti-shipping model. The Mitsubishi Ha-211 radial engine was considered at first to power the Ki-93 but both models were ultimately powered by two Mitsubishi Ha-214 18-cylinder, air-cooled radial engines, each providing a maximum of 2,400 hp. In order to give the aircraft a measure of survivability in the face of enemy fire, armour plating was used. The pilot was provided with five armour plates, each 12 mm thick. Two plates were placed just forward of the cockpit in the nose, one on each side of the pilot and the fifth would protect his back. The front glazing was composed of 70mm thick bullet proof glass. The rear gunner was also protected by a 12mm armour plate, offering defence from rounds being fired at the Ki-93 from behind. Likewise, the fuselage fuel tanks were given a measure of protection from incoming fire via an 8mm thick armour plate. Each engine was also provided with armour plating in the nacelles. Should the armour protecting the fuel tanks be penetrated, each tank was self-sealing and, to prevent fuel fires, had an automatic fire extinguishing system. Finally, a defensive armament, consisting of a single 12.7mm Ho-103 machine gun was fitted in a rear firing position to be operated by the second crewman."
-- "The difference in the two versions was in the offensive weapon fits, both mounted in ventral gondolas. The Ki-93-I Ko was equipped with a powerful 57mm Ho-401 cannon and this was backed up by two 20mm Ho-5 cannons (although one initial design did away with the two Ho-5 cannons and used a single 37mm cannon with 40 rounds of ammunition). It was anticipated that the Ho-401 cannon would inflict enough damage with a single hit to cripple or shoot down a B-29. The Ho-401 could fire 90 rounds per minute with a muzzle velocity of 518.2 m/s (1,700 ft/sec). For the Ki-93-I Otsu, the large 75mm Type 88 cannon was fitted. The weapon was an adaptation of the Type 88 anti-aircraft gun that had been modified for use on aircraft. Besides the Ki-93, this weapon was also used operationally in the Mitsubishi Ki-109 (flown by the 107th Sentai). The Type 88 had to be hand loaded by the second crewman. In addition to the cannon the Ki-93-I Otsu would carry two 250kg (551 lb) bombs."
-- "Rikugun had Dai-Ichi Rikugun Kokusho, located in Tachikawa (which is about 24 miles from the centre of Tokyo), construct the Ki-93. The first prototype in the Ki-93-I Ko configuration was completed by April 1945. In the same month the aircraft successfully took to the air making it the first Rikugun aircraft to be built and flown. However, further flight testing was hampered by the war situation, so much so that the test program was never completed. Despite the worsening conditions in Japan and delays with the flights of the first prototype the second aircraft in the Ki-93-I Otsu configuration was completed. However, it would never fly."
-- "With the surrender of Japan, the Ki-93 would become the last heavy fighter and ground attack aircraft to be built during the war."
Armament
-- One 57mm Ho-401 cannon with 20 rounds of ammunition, two 20mm Ho-5 cannons with 300 rounds of ammunition per gun and one 12.7mm Ho-103 machine gun with 400 rounds of ammunition (Ki-93-I Ko).
-- One 75mm Type 88 cannon, one 12.7mm Ho-103 machine gun and two 250 kg (551 lb) bombs (Ki-93-I Otsu).
Note that while Francillon and Dyer mention the amount of rounds available per gun, neither has anything to say about ammo available for the 75mm Type 88. Francillon claims that the Ki-109 carried 15 shells for the 75mm Type 88, while equipped with two 1,900 hp Mitsubishi Ha-104 radials, and that it was capable of 550 km/h at 6090m (342 mph at 19,980 ft) with a range of 2,200 km (1,367 miles), but no technical information is provided for rate of climb, only narrative that it was found lacking, and that weight reduction was a priority, and even RATO was tried and abandoned. This gives cause to wonder -- if the 75mm Type 88 was truly to be used against bombers, and not shipping, as (you and/or website) claim, then one way to substantiate that would be to look at the practical realities of climb rate, performance at altitude, and ammo available as used on Ki-93.
As for smaller points about the intended use for the 57mm or 75mm, it should be possible to eliminate any ambiguity by referencing also:
Robert C. Mikesh, Japanese Aircraft Equipment: 1940-1945, 2004 Schiffer Publishing -- ISBN 978-0764320972
and
Anthony G. Williams & Dr. Emmanuel Gustin, Flying Guns of World War II - Development of Aircraft Guns, Ammunition and Installations 1933-1945, 2003 Airlife Publishing, reprinted 2014 -- ISBN 978-84037-227-4
From Williams & Gustin:
-- "The IJA made some use of a 37mm Type 94 tank gun (37 x 134 R case) and a 75mm cannon, the high-velocity Type 88 AA gun, firing a 75 x 497R cartridge. It appears that the loading cycle for these weapons may have been partially automated. One of the largest airborne guns to be tested in flight was, somewhat improbably, a 102mm Italian naval gun installed in some Piaggio P.108A bombers for anti-shipping purposes. This is sometimes incorrectly described as the Ansaldo 90/53 (that was a high-velocity 53-calibre 90mm AA gun), but was in fact a 35-calibre WWI vintage piece made by either Schneider-Armstrong or Terni (both made guns to the same specification). The gun used a 102 x 632 cartridge, firing a 13.74 kg shell at 750 m/sec, and weighed 1,220 kg." (p.34)
From Williams & Gustin:
-- "The Japanese made some use of several different large-caliber cannon, although in this case they were all developed for the IJA. The Ho-203 was a Kawamura-designed long-recoil gun, firing a low-velocity 37 x 112R cartridge (taken from the 37mm Type 11 infantry gun) from a hoop-shaped 15-round 'captive belt', very similar to the 30-round magazine used with the US 37mm M4 (magazine sizes of 18 and 25 rounds were also reported). Rate of fire was only 120 rpm. The H0-401 was essentially the same design, scaled up to take a 57 x 121R cartridge (from a tank gun) and with a similar type of 17-round magazine, but rate of fire further reduced to 80 rpm. The Ho-204 was a different weapon altogether; it was the biggest of the Browning-pattern short-recoil designs and fired 37 x 144 ammunition at 300-400 rpm from a belt feed." (p.34)
-- "Experimental Japanese weapons which did not enter service included the 57mm Ho-402, a large cannon weighing 400kg and firing its 2.7kg projectiles at 700 m/s and 80 rpm, and the 75mm Ho-501 which weighed 450 kg, had a muzzle velocity of just under 500 m/s and an RoF of 80 rpm." (p.34)
Data per Williams & Gustin:
-- 75mm M4, M5 (75 x 350R) -- United States of America -- High Explosive, weight 6,670 grams, muzzle velocity 600 m/s, muzzle energy 1,200K Joules
-- 75mm Type 88 (75 x 497R) -- Imperial Japanese Army -- High Explosive, weight 6,490 grams, muzzle velocity 720 m/s, muzzle energy 1,680K Joules
-- 75mm PaK 40, BK 7.5 (75 x 714R) -- Germany -- Armor Piercing, weight 6,800 grams, muzzle velocity 790 m/s, muzzle energy 2,120K Joules
^ a comparison of three 75mm's.
There is still the 3rd source, in French, by Bernard Millot. I'm not super fluent in French but with some help from Google/Bing translators line-by-line it should be possible to corroborate or refute the sources in English.
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