Experimental Type 18 20mm Fixed Machine Gun
The "Experimental Type 18 20mm Fixed Machine Gun" was developed as a scaled-down version of the Type 5 30mm Machine Gun. It was a high-performance weapon, weighing approximately 40 kg, with a muzzle velocity of 900 m/s (1.2 times that of the Type 99 Model 2 gun) and a firing rate of 700 rounds per minute. The required performance specifications were established in September of Showa 18 (1943), shortly after the completion of the first prototype of the Experimental Type 17 30mm Fixed Machine Gun.
The ammunition was shared with the Type 99 20mm Fixed Machine Gun. Although the cartridge case was shortened by 3 mm compared to the Type 99 Model 2 gun, the propellant packing density was increased, raising the propellant charge from 21.6 g to 34 g. Design and prototyping were conducted by the Naval Air Technical Arsenal, and the first prototype was completed in January of Showa 19 (1944). Aerial testing using a Zero fighter was carried out in August of the same year.
However, by December, the practical deployment of the Type 99 20mm Model 2 Gun Mark 5, which achieved a firing rate of 720 rounds per minute, was on the horizon. As a result, by around March of Showa 20 (1945), the prototype development was effectively halted and eventually canceled.
Recollections from involved personnel indicate that the development of the Type 5 30mm Machine Gun benefited from insights gained during this project. In 1989, a prototype of the Experimental Type 18 gun was discovered at the Kentucky Military History Museum, alongside a Type 5 30mm Fixed Machine Gun Model 1, serial number 1724, manufactured by the Toyokawa Naval Arsenal.