War flag of the Imperial Japanese Army

An unusual Japanese AA Weapon

The Quarterly Newsletter of ‘The Ordnance Society‘ has been carrying a series of short four or five page illustrated articles on Imperial Japanese weapons of WWII, at least the more unusual ones. In Numbers 116 and 117 the suicide ‘lunge-mine’ and the incredible 70mm anti-aircraft barrage mortar are featured (I made one of the latter following C.O. Ellis’ brilliant, instructive articles in Airfix Magazine over fifty years ago). The most recent issue deals with a weapon I had never heard of — Japanese cyanide grenades. The series, written by Peter McAllister, is excellent and is set to continue in future issues. As a wargamer I find the content intriguing and valuable – something to be aware of if you field an Imperial army of the period.

The 70mm Anti-Aircraft Barrage Mortar (7cm Uchlaqe Sosoku-Dan)

During the history of warfare many combatants, from all periods, came up with ideas that worked far better in theory than they did in actual reality. The Imperial Japanese Army (IJA) during WWII was no exception. One of the many ideas the weapon designers of Japan came up with was a rather clever type of anti-aircraft mortar. A surprising amount of thought went into this weapon, available in two calibres, 70mm and 8lmm. I will take a look at the far more common example, the 70mm design.

Drawing of the 70mm Anti-Aircraft Barrage Mortar
70mm Anti-Aircraft Barrage Mortar

The IJA had a fairly sophisticated array of anti-aircraft weapons, aiming systems and detection devices. Why they thought, then, that they needed something as strange as an anti-aircraft mortar is, at first glance, a bit of a mystery. However, at closer inspection it can be seen that there was a strange current running through at least some of the IJA’s weapon design process. One only has to look at the hopper fed Type 11 light machine gun or the mass of (mostly unused) accessories that came along with some manufacturing runs of the Type 99 rifle. And let’s not even discuss the unneeded design of the Type 2 paratrooper rifle. Each of the above examples has something in common with the 70mm anti-aircraft mortar. In theory, they were good ideas but in practice were at least an irritant to the user – if not worse. In short, the IJA infantry’s having a light anti-aircraft weapon on hand in all terrain was a good idea but making that weapon a mortar? Not so much.

The 70mm version of the anti-aircraft barrage mortar was made (starting in 1942) at the Number 1 Army arsenal in Tokyo. As this arsenal already made 70mm mortar barrels for more conventional mortars, presumably the same facilities were used for the barrage mortar. The idea was that the mortar would discharge its projectile which, at its maximum ceiling would eject seven smaller projectiles. Connected to small parachutes these would detonate on enemy aircraft flying at low altitude. To be effective mass barrages would be needed but, despite the fairly widespread issue of the weapon, its use in its intended role was rather ad hoc. It seems that improvised platoons were its most common form of deployment. One such platoon in the Philippines had 31 men operating a mere five devices. What such an under-strength unit was supposed to achieve is anyone’s guess.

The mortar had a smoothbore barrel that was 48 inches long. The barrel was connected to a wooden block described in an American intelligence document of December 1943.

‘The base of the 70mm barrage mortar is a wooden block approximately 10 by 12 by 8 inches. Two bolts fasten a small base plate to the block. The wooden block absorbs the shock of firing and prevents the mortar from embedding itself in the ground’

A later American intelligence document, from March 1945 confirms much of what was written in the 1943 document,

‘The Japanese 70mm barrage mortar was first encountered on Attu. It consists of a smoothbore tube, 4 feet long, the steel plate of which is fastened by two bolts to a wooden block…’

The overall length is 75 inches. At the bottom of the wooden block is a long iron spike. The weapon is prepared and pointed toward the enemy aircraft by embedding this iron spike into the earth. Thus it can be seen that aiming was rather crudely done. The previously quoted document from 1943 simply says,

‘the 70mm or 81mm tube had no settings, controls or adjustments.’

This is a simple and, no doubt, cheaply made weapon. To fire the weapon the projectile was simply dropped down the barrel. If the round failed to fire the whole weapon would be slowly lifted up and gently tilted forwards to allow the round to slowly slide out. As for the projectile, it was as inventive as anything else devised during the war. It’s just a pity (at least for the IJA) that this inventiveness failed to find a better outlet. The same American intelligence document from December 1943 has a good description of the ammunition, presumably examples taken at Attu.

‘Ammunition for the 70mm barrage mortar is packed 10 to a box. The shell contains 7 parachute bombs 3 inches long by 11/16ths of an inch in diameter. A steel cylinder encases the whole assembly. The shell is painted black and is 11 9/16ths inches long and 2.34 inches in diameter. The nose is capped with a wooden disk. After the shell is projected from the mortar by the propelling charge in the base, a time train and fixed powder charge cause the projection of the seven smaller bombs borne by rice paper parachutes. At the same time a larger parachute is opened – tilting the main container and thus ensuring the scattering of the seven bombs.

These small bombs are loaded with three pellets of nitrostarch and are detonated in the air by a sensitive pull-igniter fuze with a phosphorus-coated string and delay element. They may also be used as an effective booby trap for any curious or unwary soldier.’

Again, the later American intelligence document from 1945 confirms much of the earlier intelligence document’s observations with one bonus – the 1945 document includes actual American test information.

‘Five rounds have been fired in a test, with the mortar malfunction of the delay train ignition (ed. – sadly, this is not elaborated on). The shells were quite noisy in flight and tumbled considerably, with the smoke of the black powder delay train clearly visible.

The releasing burst occurred in 7 to 8 seconds at altitudes of 1,520 to 1,660 feet and the shell cases hit the ground close to the firing position. All inert components of the round drifted to the ground within 30 seconds and the bombs drifted nearly half a mile, landing at intervals of about 30 yards.’

An example of a projectile for the barrage mortar that came up for sale some years ago (2007) was painted black with two white bands at the forward end and a red band at the other. The inside of the casing bottom still had some coiled fuse in place. The black painted projectile had a number of markings. On one side was a roughly applied area of white paint, almost a smudge, on which, in black, was the Kanji for ‘east’. The other side had a seven stage, top to bottom, series of Kanji symbols. While this is not an exact they, from top to bottom, translate as ‘seven, military measurement, together, launch, to block(?), to protect the fortress and bullet.’ As noted this may not be an exact translation. Other technical data differs from that already given. At least one modern claim says that the explosive component was RDX and the booster was lead azide. Of course, it’s very possible different types of explosive were used at different times.

It seems that the projectiles could also be fired more conventionally from the standard IJA model 11 70mm mortar. Though it is obscure as to what effect that tactic had on the battle field. Very oddly there is at least one eyewitness case of the barrage mortar being mounted in a Japanese bomber for defence against allied fighters.

The rice paper parachutes were around a foot in diameter, perhaps in some cases a bit larger. It is also clear that black powder could be used instead of the more usual ignition sources. As for maximum range, one American report gives the fairly unlikely number of 4,000 feet. Between 1,000 and 2,000 feet was far more realistic.

The blast radius, despite the small charge of the individual bomblets was around a 10 to 20 yards radius.

Stripping the 70mm barrage mortar was easy. First the barrel was unscrewed from the metal base plates, thus separating it from the wooden base block. The firing pin might then be removed from the fitting that holds it to the base plate. Finally the iron spike is removed from the wooden block.

On a last note, a May 1944 American intelligence report is fairly blunt about the weapon’s prospects in battle,

‘Although no instance has ever been reported of our aircraft being damaged with this weapon, it would appear that this weapon might be very effective against low flying aircraft if used in sufficient quantity.’

As previously noted however, these weapons tended to be used in penny packets. The fact that those issued with them sought to find other uses for the projectiles speaks for itself. In short the 70mm (and 81 mm) anti-aircraft barrage mortars must be considered interesting failures. Before those of British heritage become too smug however, a similar British project did catch the eye of Winston Churchill. Thankfully, cooler heads made sure it came to nothing.

Article by Rob Morgan.

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1 thought on “An unusual Japanese AA Weapon

  1. Hi I’m the Peter McAllister who writes those articles. More than happy to have them reproduced by you.
    The second part of the cyanide article will hopefully be out soon. It concentrates on a actual skirmish in which they were used in 1942 against British tanks of the 7th hussars. Apologies for the long delay.

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