That IS very interesting. One has to wonder how accurate it is though; sometimes you can't tell whether they had it right at the time, or the modern references are the accurate ones. This listing is for a "Sally II", i.e. one of the later types with the hemispherical dorsal turret, not the one I am most interested it, which is the early "greenhouse" type with the machine gun deployed from a hatch (I didn't specify this however, and it may make little or no difference to the tail armament used). But above this listing, they mention a "Sally I", which they describe as being armed with a "20mm cannon" in the tail stinger. None of the references, pictures or drawings I've seen agree with this; they universally claim a 7.7mm machine gun, which appears to be a Type 97 complete with barrel shroud. (The belt-fed type that the Zero fighter uses, not the Bren-gun style Type 97 infantry gun). It is, however, entirely possible that some Sallies were re-fitted with one of the several different Japanese 20mm cannons in the field, or even from the factory. There is a sad lack in surviving information about Japanese aircraft, particularly info that is easy to find, accurate, or both. The more I read about them, the worse it gets. So much is contradictory, you frequently find information that is flat-out wrong even when you CAN find it, and most of it is just quick references here and there. I've just about lost faith in Wikipedia for anything regarding the topic of Japanese weaponry.
But I digress. While searching, I've also found another reference to the tail gun, which agree with the dorsal-position idea. At:
http://www.j-aircraft.com/faq/ki21.htm they refer to a "tail gun controlled from the dorsal position as well". Still lacking in technical details, which is what I really lust for. Perhaps it's gone forever; there's a lot of stuff that people seem to just not know for sure, since the US military (so wisely...) scrapped every single example of most Japanese types after the war.
Anyway, the "20deg field of fire" and "reflector sight" are a great clue, and I thank you for that. Unless they are totally off (possible considering the "20mm cannon" claim...this was wartime after all), it certainly indicates a true remote control tail gun.
I would say, however, that the "reflector sight" reference probably doesn't mean a sight on the gun ITSELF, rather that there was a separate, remote sight within the dorsal position, much like B-29 or Me 410 used. In this case, it was likely a simple reflector sight like early war fighter-types used, where a "pipper" is projected onto a pane of glass, rather than the complex analog-computer sights that later war fighter aircraft and the B-29 used, which automatically corrects for range, speed and deflection (not sure about the Me 410). With a hand-aimed machine gun, the distance rear and front sights gives you an acceptable "sight-radius", or distance between the two sights. The shorter the distance, the more any slight variation in aim is magnified. You could set up a remote sight using a long rod as a stand-in "gun", where the remote gun automatically moves to point exactly where the rod is aiming, but you'd have to make the rod a foot or two in length in order to get an acceptable sight radius. Or you can use a reflector sight, which just like a "red-dot" sight today is parallax-free, meaning that when you look at it, wherever the "pipper" is, that's where the gun is aiming (not countering for bullet drop or deflection). This means you can use a pane of glass and a couple grips to aim the remote gun. It's far more compact, especially in a position already containing a REAL gun. The other benefit is that it's far easier to set up a computing sight using an easily-moved "pipper" on a pane of glass than an actual sight. They tried it, it doesn't work! (Note: the B-17 used a semi-remote "rod" with sights mounted to aim the rear guns, but this was mechanically linked to match the actual guns directly below, which is far different from a true remote mounting...still, it can't have helped accuracy.)
In any case, I envision a position where the dorsal gunner is given both a handheld Type 89 machine gun AND a separate, compact reflector sight to aim the tail stinger gun for when the vertical stabilizer is obstructing his view. The questions now are:
"how was the sight linked to the rear gun?",
"was this on all Ki-21's or just the 'Sally II' version?",
"was the gun in question a 20mm or 7.7mm, and which type was it specifically?"
"what was the ammunition capacity, etc?"
"what problems arose in use, and why is this not a better-known feature, considering that it's far more novel than anything I've seen in just about any other aircraft at the time?"
This was a time when most aircraft were using ventral-tunnel armaments or periscopes to aim ventral "turrets", or forgoing them altogether. The early B-17's lacked tail guns at all. I've never seen a remote-control system that was even STARTED by the US until after the war began (I'll have to check though...I have a VERY detailed book on US armament systems somewhere around here). In any case, once again I can't help but wonder if Japan was really so technologically backwards as typical accounts suggest. If they were behind, it wasn't for lack of trying. The older I get, the more I wonder at how obscure Japanese WWII aviation really is. Sure, they lost, but so didn't the Luftwaffe...and look at THEM!
