[philman_36]

Do the same thing as with the Warthog...build the aircraft around the weapon.

For shore battery powder guns are going bye bye, IMO.
Railguns as the weapon system and build the ship around that.
Forward and aft battery, close support (Vulcan, BPDMS or similar)
It could be done smaller than 20,000 ton and be more effective/destructive.

[philman_36]

The Navy's Railgun Is About to Get Faster and More PowerfulThe railgun works by using extremely high electrical currents to generate magnetic fields capable of accelerating a projectile to speeds of up to Mach 6, more than twice as fast as existing projectiles. The railgun has a range of more than 100 miles. It fires projectiles that destroy targets not with high explosive, but by smashing into them at hypersonic speeds.