From: St.Petersburg, Russia
I've started working on subs.
Meanwhile, I had some thoughts from another area, I thought I should write down.
I was thinking here, if it was realistically possible for Japanese to seriously improve their shipboard AAA capabilities, and what can be possibly done in this area either in Scen 70, or in relatvely realistic mods in general. Unfortunately, the main issue actually is not some identifiable mistakes, but Japan simply being too poor and underdeveloped to keep up with the Allies. This is well-illustrated by...
- Small-calibre AAA. Japanese 25mm is usually maligned, but what what actual alternatives they had? Versions of Bofors and Oerlikon guns that gave USN the edge in anti-aicraft defense do not and will not even exist before the war. If we imagine that Japanese buy, say, a license for the Bofors gun as it existed before the war... they won't get anywhere close to US quadruple water-cooled Bofors mount. They will get a gun with good ballistics, which mount they will need to cram in the same weight category as their 25mm triple mount, so it can be operated with the same electric motors as 25mm powered mounts (we don't postulate overall massive boost to the economy yes?), or manually, which is necessary, as most 25mm mounts were, by necessity, manually operated. So, it will, by necessity, be placed in single mounts. And then the combination of low maximal rate of fire, small magazine and air cooling will eat most of the benefits of the good ballistics through inadequate practical rate of fire. Even if the industry will be able to handle it in the first place, which, looking at the quality of wartime Japanese Bofors clone, is far from a given. Soviet 37mm Bofors derivative 70-K is probably a good approximae example of what Japanese could have actually created, and it was a very mediocre gun, for the reasons stated above.
With Oerlikon the situation is even worse, as existing versions Japanese could have licensed were unsuitable for real mass production and/or godawfully bad, particularly at the time when Japanese actually adopted Hotchkiss 25mm. The only decent Oerlikon derivative, save for US Oerlikon revorking, German C/38 Flakvierling, did not exist until 1940, far past the point where Japanese were choosing a new AA machinecannon. And to be honest, all of them 20mm guns were grudge weapons anyway, as far as large warships are concerned.
And, by the way, what other fleets had at the end of 1941? British 40mm Vickers was really bad, with pathetic effective range. In fact, that's why Japanese already phased this gun out. By the end of the war it was made kinda-sorta effective due to advances in mounts, fire control and whatever, but in 1942 British shipboard flak failed hard indeed against Japanese.
USN 28mm gun is not measurably better than Japanese 25mm, and also is unreliable. Its only serious advantage is far more powerful motors, allowing to place 4 barrels on a single mount and giving it better rates of elevation and train, despite far greater weight. Which, AFAIK, also made it unsuitable for manual operation, and most of Japanese 25mm mounts were, by necessity, operated manually. As it is unreasonable to expect from Japanese to radically improve their motor production... But no one offers them that gun anyway.
Germans had very poor 37mm non-automatic guns and above-mentioned 20mm C/38, which was a good complementary weapon to something more powerful, but inadequate by itself. They, AFAIK, had no powered mounts for small-calibre AAA whatsoever. They compensated with good fire control and large number of guns... and not facing the level of challenge IJN did.
Don't really know about Italian fleet, but I somehow don't think they had anything awesome...
So, basically, Japanese picked if not the best, then one of the best available options, and were at least on par with other major fleets before PH in this area... and after the war actually began, Americans one-upped them by creating practically a new generation of weapons, while IJN was stuck with what it had, because Japanese economy or research capability was nowhere near as powerful.
Short version: There was no magic choice that can suddenly improve IJN's small-calibre AAA situation. I can speculate about what could have been realistically done to make 25mm a better weapon, but this will be rather presumptions, considering how superficial my knowledge is. And considering that one of the most severe problems with Japanese 25mm guns, if not the most severe one, was their insufficient numbers until late 1944, I don't know from where they could have scrounged enough yen for technical improvements here...
- Medium calibre AAA/DP guns. Well, here Japan was mostly in OK position, save for, again, not really having enough of them until way too late, not having enough high-power mounts until way too late, not having enough modern fire directors, and so on. And here we also more room for speculations and what ifs, considering the range of existing options. I think the most obvious improvement here is using either 127/40 or 120/45, but not both (type 89 mounts could have just used 120/45 with the same level of success, type 88 mounts could have replaced 120/45 singles...). The former has heavier shell and greater explosion envelope, the latter slightly better ballistics... they are about equal to each other. Unfortunately, it is unclear if 127/50 DP Japanese were developing had any promise for shipboard use, so unifying all weapons to 127mm is a weak idea, if it forces Japan to abandon 100/65. A more exotic, but, IMO, viable option will be using 100/50 gun (historically used only on a couple of subs) in place of 127/40 on type 89 mounts, until 100/65 is developed. 100/50 had even better ballistics than 120/45 and decent possible rate of fire.
Of course, all of those alternatives, save of not making any more 120/45 during the war, phasing them out in favor of more 127/40, require the point of divergence at the beginning of 1930s, so they are not really applicable to Scen 70.
Another possible alternative is using 76/60 gun as an intermediate weapon between DP guns and small calibre, and trying to develop a semi-automatic gun on its base. I've seen it posed before, but, unfortunately, cannot really comment on its technical viability, except that lack of interest in it IRL most likely indicates lack of such. Japanese themselves considered 76/60 inferior to 100/65 and wanted to rearm surviving Aganos with the latter, but never actually got to that.
Whew. To think I wrote all of that for the conclusion like "Japanese AA weapons should probably stay as they already are, for this scenario".