13.2-mm Aircraft and Antiaircraft Machine Gun Ammunition20-mm High-Explosive Projectile
Japanese Explosive Ordnance – Army Ammunition – Navy Ammunition
Chapter 5 - Section 1
NAVY PROJECTILES
Type 99 - 20-mm Ammunition

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.

Case

Mk I

Mk II

 

Length

3.79 inches

3.98 inches.

 

Diameter at base

0.75 inch

0.75 inch.

 

Weight (empty)

57.0 grams

71.0 grams.

 

Material

Brass

Brass.

 

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:

Original type: No bands.
Modifiaction 1: One 5-mm white band.
Modification 2: Two 5-mm white bands.
Modification 3: Three 5-mm white bands.
Modification 4: One 10-mm white band.
All the Swiss projectiles are painted a solid color without bands.
The following dimensions are common the all projectiles:
Distance from rotating band to bourrelet, 1.29 inches.
Distance from rotating band to base, 0.36 inch.
Diameter of body, 0.77 inch.
Diameter at bourrelet, 0.79 inch.
Diameter at base, 0.75 inch.
Diameter at rotating band, 0.83 inch.

Figure 349 – Type 99 20-mm Ammunition.

13.2-mm Aircraft and Antiaircraft Machine Gun Ammunition20-mm High-Explosive Projectile