Aircraft gun
Desperate War of Japan in the Pacific
 12.7mm & 20mm
Independent development
1943IJNType 3
20mm1939IJNType 99
1945IJNType 5
US and Japan did a long air battle in New Guinea and the Solomon Islands almost two years. Logistics was critical for both nations to continue battles. USN and USAAF had adopted the same 12.7mm gun for aircraft. IJA fighters aircraft flew to the battle front of the south east Pacific. Ammo shortage troubled IJA fighters at Rabaul seriously. But IJN could not supply with ammos because of no compatibility between IJA and IJN, though they had modified M2 gun independent. IJA succeeded in explosive charge of 12.7mm at fisrt. Why did IJN develop another poor 12.7mm gun in 3 years later? On the contray, IJA could not make much ammo of 20mm gun till March 1944. It was 5 years later after IJN production of Type 99 cannon. And worse production of explosives also was very little compared with the US.
Rikugun-ki tousai kikan-jyu
Danyaku no kikaku

Fig.2 IJA 12.7mm ammoFig.1 IJN 20mm gun

Fig.1 shows 20mm cannon of IJN Type 99 and its ammo. IJN purchased manufacturing licence from Oericon in 1938. Oericon of Switzland developed it in 1934. IJN imported all the machine tools and materials. USN also adopted it for close anti air gun of warships, though USN adopted famous Bofors 40mm cannon later. Fig.2 shows IJA Ma-103 of 12.7mm ammo which was developed at the First Tokyo Arsenal in 1940. It was originated from Breda in Italy. Italy is a cannon country since Renaissance. Ottoman Empire loved Italian guns. JMSDF uses Italian rapid fire cannons now. JMSDF and JAFSDF use the same ammos compatible US now. Fig3 shows IJN and IJA main ammos.

IJN fighter A7M2 was to eqiupped with 30mm guns. An IJN arsenal made 2,000 guns of 30mm. But The fighter was not in time to production. While IJA fighter Ki-84 model hei had prepared for 30mm guns. IJA ordered Nihon Tokusyu Steel Co. to design a trial of 30mm gun in 1940. The fihgter was to load Ho-155 of 30mm guns which were not in time to production.
Gunzo no.89 p.81-89
Kouku kijyu

Ammo1. IJN 7.7mm
2. IJN 12.7mm
3. IJN 20mm, Type99 model1
4. IJN 20mm, Type99 model2
5. IJA 20mm, Ho-3
6. IJA 20mm, Ho-5
7. IJA 20mm MG151
8. IJA 12.7mm, Ho-103
9. IJA 7.7mm
Fig.3 Ammos of IJN and IJA
Bureaucracy of IJA and IJN prevented cooperation with each other, I think. It concerned a lot of budget. IJN and IJA managed arsenals in which ammos were producted. The arsenals spent a lot of money. There were occupations for high ranked officers in the arsenals. I think this is one of reasons why IJN and IJA developed ammos for aircraft guns independent. We say even now, "Shou-eki atte, koku-eki nasi. (For ministry first, national next)" I wish ammos of JMSDF and Japan Coast Guard are compatible.

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