Developed in the 1930s, the Type 99 was the go-to 20mm autocannon for Japanese military aircraft, serving as the wing guns, flexible mounts and powered turret mounts on a number of Japanese fighters and bombers, including the A6M "Zero" carrier-launched fighter of Pearl Harbor fame. Like many combatants on both sides of the Axis/Allies divide, the Japanese design directly follows the pattern established by the Oerlikon autocannon, using an advanced primer ignition (API) mechanism to reduce the amount of breech locking needed to fire the potent 20mm cannon round while also permitting a high rate of fire. This Mark 1 cannon is configured for the flexible defensive mount role, with the mounting point for the (not included) ammo drum on the underside to keep it out of the gunner's line of sight, and an ejector port shroud to direct the spent casings to the shooter's right. A heat shield assembly protects the back half of the barrel, the recoil spring, and the bolt extensions, with a ratcheting hand-cracked charging mechanism on the right side. The left side of the trigger housing bears the Japanese model nomenclature, with dual pistol grips and a slot on the back for installing a (not included) buttpad. Sights absent.