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 Day Battle or Night Battle.

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

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. It is not known whether the bonus applies to ALL 20.3cm guns or a select few.

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.