I worked on a house rule for coastal guns almost a year ago, it went something like this…. (BTW… I’ve given up on trying to rid this global forum of house rule discussions).
COASTAL GUNS
Coastal Guns cost $9 each and can be immediately place on any territory that has a least one adjacent sea zone. When there, each coastal gun will fire a maximum 3 shots @1 to hit any enemy surface war ships attempting an amphibious assault from any sea zone adjacent to a territory with coastal guns.
-The Amphibious landing does not have to target the territory with the guns in order to be fired upon (same as scramble rules).
-The maximum 3 shots per gun only applies if there are a sufficient number of surface war ships to fire upon (same as AA gun rules).
-Nations may purchase and place as many guns per territory as they wish (same as AA gun rules)
-Costal guns may only be used when the enemy is attempting an amphibious assault, and they fire before the sea combat sequence begins.
-Costal guns may never fire upon transports or submarines (or you can also use the “last casualty rule” for transports and subs).