For your interest...
This is a summary of a report I did for Air Warrior way back when... please correct me if I am wrong...
Sources:"Fighter: Shidenkai" by Y. Ikari, 1975
"Reisen, A small history of the Japanese Naval Aviation" (Japanese book, I forget the details)
ISBN4-7698-0631-0 "Shiden, Shidenkai, and Type94 Float Plane" edited by Magazine Maru Editorial Department 1992
ISBN4-499-22608-2 "Aero Detail 7 Mitsubishi A6M Zero Fighter" by S. Nohara 1993
ISBN4-499-22711-9 Kawanishi N1K2-J "George" Shiden-Kai
On the Type 99 cannons:The difference between the Type 99-1 cannon and the Type 99-2 cannon is the length of the barrel, Type 99-1 being lighter by 10kg. Muzzle velocity differed by 150m/s which is 600m/s and 750m/s respectively. Ballistics were that when the shells travel 500 meters, they drop 1.5 meters for the Type99-1 and for the Type 99-2, it was half of the Type 99-1.
The Type 99-1 Model III had drum type magazine with 60 rounds. Type 99-2 Model III had drum type magazine with 100 rounds. Type 99-2 Model IV had belt-fed system.
On the A6Ms:A6M2 had the Type 99-1 Model III with 60 rounds each in favor of the 10kg difference in weight. It says in the book "Reisen, A small history of the Japanese Naval Aviation", that from the fourth plane in production, they switched to a magazine with 100 shells. Were they able to enlarge the drum type magazine without changing the gun itself... I donno... The first plane of this type was in production from December 1939 and 64 were produced at the Mitsubishi plant. This was the Model 11.
The wing folding type of the A6M2 (740 planes at the Mitsubishi plant) for the CVs were in production from 1940. This was the Model 21, which made itself known in the Pacific War.
A6M3 Model 32 went into production July 1941. A6M3 Model 22 went into production in autumn 1942. In this model, the cannons changed to the Type99-2 Model III in favor of the ballistics, and the planes with these cannons were called the A6M3a.
The next version is the A6M5 Model 52 which went into production summer of 1943, 747 being produced at the Mitsubishi plant. The table of the Zekes in the book lists no modification on the guns from the A6M3. So, I think the cannons for the A6M5 was still the Type99-2 Model III with a 100 shell magazine.
The belt type Type99-2 Model IV was first used on the A6M5a which was first built in autumn 1943, with an increase of shells to 125 per cannon. 391 planes of this type was produced in the Mitsubishi plant.
On the N1Ks:The Shiden (N1K1-J) was fitted with two Type99-2 Model III with drum type magazine of 100 rounds each and two 7.7mm machine guns which was the case with the Kyofu (N1K1), the float plane where the Shiden originated from. The N1K1-Ja had the two 7.7 mm machine guns replaced with two more of the Type99-2 Model III. This modification happened after the first 300 N1K1-J were built. It seems they had to build a casing attached to the wing to incorporate the extra gun on each wing, while the space for the 7.7mm still left as is. So, with this modification N1K1-J became the N1K1-Ja. The N1K1-Jb had the belt type Type99-2 Model IV.
The Shidenkai, N1K2-J, had four of the belt-fed Type99-2 Model IV from the beginning. (Post edit: I forgot to mention that the ammo load for the N1K2-J is 200 rounds for the inner guns and 250 rounds for the outer guns for a total of 900 rounds... This was in response to the question posted over at the Gameplay Feedback/issues forum.
Did n1k2-j really carried 900 rds of cannons??? )
Well... that's it for now...
