Naval 20-mm ammunition is divided into two classes: Type 99 Mk I and Type 99 Mk II. The two are distinguished by the fact that the Mk I uses a small-size cartridge case (3.79 in.), and the Mk II uses a large-cartridge case (3.98 in.).

Both sizes use the same projectiles and fuzes, but there are a greater number of types and modifications of rounds in the smaller size.

This ammunition is a Japanese copy of the Swiss Oerlikon design. In addition to rounds of Japanese make, some of Swiss orgin have been used by the Japanese.

The cases are of the reduced-rim type and are crimped to the projectiles by either three or six crimps at the cannelure, and sealed with a clear lacquer.

Propellant: The propellant from the Japanese-manufactured rounds is graphited, single- base, single perforated, cylindrical, nitrocellulose grains. The propellant from the Swiss rounds is single-base, graphite-coated, nitrocellulose grains, in the form of flakes. The Mk I case contains 13.6 grams of propellant and the Mk II case contains 21.4 grams of propellant.

Projectiles: In the case of Japanese manufactured ammunition, the type of projectile is indicated by painting the projectile body a solid color. Subsequent modifications of the original type are indicated by one or more white bands around the projectile body, thus:

Figure 349 – Type 99 20-mm Ammunition.