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81mm Type 100 HE Mortar Round

Japanese Type100 HE Mortar Round with Type 100 selectable impact fuze, used with the Type 97 (1937)
and later the Type 99 small trench mortar. This is based on the Stokes-Brandt design as was the U.S. 81mm, and this round was actually interchangeable with the U.S. M43A1 shell.
These had a maximum range of about 3,000 yards
Loaded weight: ~3kg (6.9lbs) of which 1 pound was TNT.

The stenciled text translates-
Left column:
"100 Type (Model)"
Right column:
"97 Infantry Mortar"

Kyoku Ho is the abbreviation of "Kyokusha Hoheiho", which the IJA called the infantry mortar.

Note: The "+" on the back side indicates it is 0.5 to 1.5 percent over standard weight.

There are 5 possible markings:
++   +    +/-    -     --    

 +/- indicates nominal weight, while ++  1.5% to 2.5% over, and -- 1.5% to 2.5% under.

Evidently this was to make up for variations in manufacturing variables. These weight indicators would be taken into account when aiming, to obtain the desired ballistics.

Fuze with Booster Cup
The Type 100 Selectable Impact Fuze can be set for instantaneous or delay function by turning a small adjustment screw. Upon impact, the nose cap is forced in, breaking the shear wire safety bringing the firing pin and primer together. The flash from the primer is either allowed to directly enter the booster through the flash channel or, depending on the orientation of the select screw, ignites a short delay train.

Diagram at right from  TM9-1985-5
U.S. Army Technical Manual,
"Japanese Explosive Ordnance."
(A really great reference.)

Fuze Diagram

Fuze Function Selector
Top: "Instantaneous" - Right: "Delay Action"

Osaka Army Arsenal 9; 8 10 Showa
(Sept 1943)

IncrementMortar Increments
These are individual propellant packets which are removable. The Type100 had six per round, but the actual number used depended on the flight ballistics required for a particular shot, removing the unnecessary ones prior to firing.
Propellant Increments

The bags are made of silk fabric and were covered with some sort of waterproofing shellac, which evidently worked very well as these were recovered from Guadalcanal in 1994 on the "Maruyama Trail", scattered in the rain forest humus near a large pile 81mm mortar rounds. They have since been emptied of their contents, some sort of flaked nitrocellulose.
(Thank you Ewan & Steve!)

The character is iwa (also pronounced gan). It means rock, crag  or reef.
There is a prefecture that starts with this character, Iwate. It is north of the main island, and there is also a city by the same name. Apparently  there are no specialized military words than start with this character, so a guess would be that this is a manufacturer's mark for the factory location, Iwate. That would make the 20 (21) possibly a batch number. The 15.12 is most likely the date, December 1940.
(Thank you Teri! - Teri's Nambu World )


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