This is how my coastal gun rule would read (nice and simple).
Coastal Guns (cardboard counter)
Cost: 9
Attack: N/A
Defense: 1
Move: N/A
Unit Characteristics
Can’t Move: Coastal guns can’t move once placed on a territory and they can’t be carried on a transport.
Sea Defense: A coastal gun may only be used during an amphibious assault from an adjacent sea zone from which the coastal gun resides. A coastal gun can only fire into one sea zone per turn, and at sea units that reside in an adjacent sea zone from which an amphibious landing is taking place. A coastal gun does not contribute to the defense of industrial complexes, airbases, or naval bases during Strategic Bombing Raids. Coastal guns fire only once, before the first round of an amphibious assault.
Roll one die against each surface warship in the adjacent sea zone from which the landing will take place (coastal guns fire after the decision to scramble, but before all combat). On a roll of “1,” that sea unit is destroyed and immediately removed from the game. If all the units are the same, there is no need to differentiate them. However, if there is a mix of warship types, you must assign specific dice rolls to specific types by indicating which surface warship is being rolled against. A roll of “1” destroys the specified sea unit (unless the sea unit is a capital ship, in which case it will require 2 hits to be destroyed).
Coastal gun defense is made immediately before normal combat occurs in the territory containing the coastal gun.
Subject to Capture: If a territory is captured, any coastal guns there is also captured (unlike AA guns which are removed from the board once captured). Their new owner is the player controlling the territory at the end of that turn, and that owner can use those coastal guns in future sea zone defense. Coastal guns are never destroyed. Only one coastal gun may occupy each territory. Ownership of the coastal gun is directly linked to ownership of the territory, And it will defend the territory it occupies no matter the type of allies present in the territory.