Combat/Overweight Penalty and Fit Gun Bonus

Battleships
According to official twitter, there won't be a "very big change" in accuracy (positive or negative) due to this system.

Putting suitable weight guns on Battleships would increase shelling accuracy in both Day Battle and Night Battle.

Putting too many heavy guns on Battleships would cause a drop in accuracy.

Third party Inspection on BB Fit Gun

 * 【艦これ】戦艦主砲の装備補正検証　その4　-フィット・過重量砲の概念は扇?　16inchMk.7など同種主砲2本積み9～11種類戦艦型別命中率分析- (Japanese)
 * 戦艦主砲の装備補正命中率検証 (Japanese)
 * Iowa改フィット@2本積み (Japanese)
 * Warspite改フィット (Japanese)

Heavy Cruisers
CA(V) equipped with 20.3cm guns would get an increase in Night Battle accuracy. When asked about if this bonus applied to enemy fleets, the developers responded that the bonus only applies to KanMusu.

Some data shows that not all 20.3cm guns enjoy the bonus to the same degree, several players are now investigating about it.

Light Cruisers
According to official twitter, using certain "Fit Guns" on Light Cruisers will improve their accuracy.

It's also known that using those Fit Guns can increase their firepowers.

Training Cruisers and Torpedo Cruisers also enjoy the relevant bonus.

Firepower
Light Cruisers, Training Cruisers, and Torpedo Cruisers enjoy firepower increase when using any of the following gun:
 * 14cm Single Gun Mount
 * 14cm Twin Gun Mount
 * 15.2cm Single Gun Mount
 * 15.2cm Twin Gun Mount
 * 15.2cm Twin Gun Mount Kai

It have been suggested that the increase in firepower might follow this relationship:
 * Increase in Firepower = √(No. of Fit Single Gun Mount) + 2 × √(No. of Fit Twin Gun Mount)

Accuracy
So far, only limited amount of tests have been conducted to study light cruiser fit gun's effect on accuracy so all information in this section are in-comprehensive and they might be superseded by other, more up-to-date investigations when those information are available.

Accuracy increase have been observed for the following guns when they are being equipped on CL Accuracy increase have been observed for the following guns when they are being equipped on CLT Accuracy increase have been observed for the following guns when they are being equipped on CT Other combinations have not been fully tested yet.
 * 14cm Single Gun Mount
 * 14cm Twin Gun Mount
 * 15.2cm Twin Gun Mount Kai
 * 14cm Twin Gun Mount
 * 14cm Twin Gun Mount

The possibility of accuracy bonus vary across different ship classes have not been ruled out yet.