Light Cruisers in Kancolle can equip 20.3cm Twin Gun Mounts without penalty. They just don't get any accuracy bonus for using Fit Guns (15.2cm). As a Dev Tweet has said, "there is no Washington Naval Treaty that limits gun caliber on warships in the game."
Some admirals actually prefer the 20.3cm on their CL for the higher combat power.
KUM-Thanks.Good work.Hey.I'll take your equipment off your hands.The newly-developed 20.3 - how was it?All hands without even so much as light damage - That's the Fighting Kuma Sisters for you.Got it kuma.Okay, can you get me a report? Later's fine.hmm... It's a bit heavy kuma. And the barrel feels kinda off kuma.Man, having such capable little sisters, I'm just so proud kuma.