Talk:5inch Single Gun Mount Mk.30/@comment-175.156.13.185-20180422161421/@comment-33100091-20180521062855

That's kinda the point, IJN gun control system generally are very reliant on visual information and they lack the ability to make any sort of accurate gunfire without clear direct visual line of sight.

This was a limitation not shared by the USN DD though their DE does not have the surface radar.

As a note, IJN CA (CL before conversion) were all intended to carry 203mm turret with 5.9 inch frontal armor on the turret, however not all of them had one as the armor weight caused them issue in their testing, which led to the changes in the design for cruisers like Mogami class.

No one knows exactly which cruiser carried which configuration, and there's no complete record of it left.

if you look on the sources for IJN CA data you might see 25mm being mentioned as their turret armor... and this might be accurate for the pre conversion spec (ie: the data that was available and intended to comply with the treaty before the war) but it doesn't really reflect the cruiser post conversion into the 203mm guns and it doesn't even necessarily reflect their actual armor before conversion because...

you might notice the weight on these cruisers on those sources...

These weight are basically almost certainly wrong as well if it's anything less than 10000 ton because by the time they were converted with the 203mm guns their weights were well in excess by several thousand tons over that limit (the addition of the new equipment and the 203mm gun to the officially reported weight before conversion doesn't add up to their estimated final weight so the official figure for pre conversion weight most likely is false and only intended to keep the appearance that  they are within the treaty limit).