User blog comment:Hayashi H/Advanced Equipment Guide/@comment-11937523-20151017182910/@comment-26486243-20151024131121

The idea is that battleships and carriers are the ones that will be killing enemy bosses in events, map-clear missions and such, the vast majority of the time.

Carriers are their own topic due to their complexity, but also don't involve Akashi so they don't affect this.

As for BBs, max accuracy at firepower cap is important. It is possible to get firepower cap using 41cms early instead of speccing into upgrades, but on Kongous and such the overweight penalties plus the lack of post-upgrade accuracy bonuses may make this a gamble if they don't hit, esp. in combined fleet settings as Zeraphix mentioned. If you have access to additional 38cms from Bismarcks, they can take the place of some upgraded 35.6cms, but because Bismarck takes a large amount of resources to craft, this may not be a good first resort.

CAs don't generally kill bosses except at night or with T3 shells; upgrading is almost as much for the day shelling accuracy so they can clear the trash reliably, as it is for the firepower itself. In the night battle case, accuracy doesn't matter as much since CAs already get an accuracy buff only in yasen with 20.3cms involved and their night battle value is so high that adding further firepower is a smaller %gain than adding it to battleships.

All rules have exceptions of course - CAs for instance were the optimal ship type to use in Summer 2015 E2 STF for instance even up to Hard difficulty as the opposing ships couldn't pierce CA-level armour, there were no ships heavier than CAs until the boss, and the boss itself can be easily handled in night battle even in final formation - upgraded 20.3s would have done wonders there. But almost every other map past that reverted to the BB/CV/Yasen general rule again.