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This last episode with the Japanese Type 99 auto cannon was interesting on several levels. The Type 99 was a modified copy of the famous Swiss Oerlikon 20mm gun. The Oerlikon was used by both Allies and Axis powers during WW2. The Type 99 came in both flexible and fixed configurations (the show used a fixed, solenoid fired gun). More problematically, the Type 99 came in two distinct versions and ammunition was NOT interchangeable. The Type 99 Mod 1 used 20x72RB (rebated rimless) ammunition and the Type 99 Mod 2 used 20x101RB (rebated rimless) ammunition. The gun could be setup to use 60 or 100-round drums or a belt feeder that replaced the drum and used linked ammunition.

The Type 99, like all Oerlikon guns, does not use a locked breech (like a .50 machine gun). Instead, it uses very heavy coil springs and is blowback operated using advanced primer ignition as an operating principle. TIMING is extremely crucial to the operation of this gun because the chambered cartridge fires as the bolt closes. The inertia of the forward moving bolt and driving springs keeps the breech sealed until the chamber pressure drops to safe levels.

Oerlikon guns, including the Type 99, do not have primary initial extraction (to loosen the fired case in the chamber). Instead, the bolt begins to recoil with a jerk that can, and has, ripped the back off cartridge cases. There are two ways to prevent this: (1) use oil or grease to lubricate the cartridge to prevent sticking (Oerlikon) or (2) cut longitudinal flutes in the chamber mouth to float the front of the cartridge case on burning propellant gas (Tokarev SVT-40 and H&K G3).

There was no discussion on the show of whether the ammunition for the Type 99 was LUBRICATED. If it wasn't there was a good bet that the fired case would have stuck in the chamber and had its case head ripped off. This would not be too much of a problem if only one round was fired, but it would have been a problem for a second round. Unless they were shooting inert TP ammunition, 20mm HE is NOT bore-safe and it could detonate in this kind of jam. Will Hayden was completely right in stopping his guys from firing the Type 99.

Trivia about the Oerlikon guns. It requires approximately 450-500 pounds of force to cock this gun due to its massive springs. On naval mountings, such as the Mk 10 tripod mountings aboard the USS KIDD (DD-661) shown on an earlier program, there was a cocking bar. To cock the gun, the gunner attached the cocking bar to a stud on the side of the gun and jumped up onto the gun's shoulder rests to use his body weight to cock the gun. If he was too small in stature or weight, the loader would push up on the barrel as the gunner was using his weight on the shoulder rests. In the episode with the Type 99, that is the reason why they fitted it with the two long steel bars on the front sides of the gun — to help cock it!