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Developed from the Swiss Oerlikon FF, the type had many similar characteristics. The weapon itself was drum fed, with each drum having the capacity for 60 rounds of 20mm ammunition.
The Type 99-1 had a total length of 1.33 meters, with an 81 centimetre barrel, and a total weight of 23 to 26 kilograms. The rate of fire was 500 rounds per minute while the muzzle velocity was around 600 meters per second. In fact, this was one of the main drawbacks of the weapon, a relatively slow muzzle velocity compared to contemporary designs. The type of ammunition used varied from incindiary to high explosive but all were fired in the 20x72mm RB Cartridge.
The first variants of the original gun were the Type 99-1 Models 2-4. These were all quite similar in design except each had their own modifications. The Model 4 for example was belt fed instead of using drum magazines, which simplified the process of firing dramatically while the Type 3 was given a 100 round drum magazine.
The Type 99-2, which had the main distinction of being a heavily upgraded and upgunned version of its predecessor, was chambered for the 20x101 RB Cartridge, and had a total length of 1.90 metres, with a 125 centimetre barrel, total weight of 34 to 38 kilograms, and rate of fire of 490 rounds per minute. It was more along the lines of a similar off-shoot of the original gun, but was still technically in the same series as the 99-1. Like it's predecessor, it had several sub-variants, each possessing their own characteristics. The Model 4 had a belt fed mechanism like its Type 99-1 counterpart and the Model 5 had an increased rate of fire to 750 rounds per minute.
The Type 99 was developed in 1938 and entered into service in 1939, becoming the principal 20mm autocannon of the Imperial Japanese Navy. It was constantly being improved and revised and served its users well. Furthermore, the type had been adapted for service in a multitude of different platforms. These include the Mitsubishi A6M Rei-sen, the Kawanishi H6K, the Kawanishi H8K, Kawanishi N1K Kyōfū, and Mitsubishi G4M among others. In total, thousands of Type 99s had been produced for the Navy.