I'm an avid battleship builder, and 4 Rocket Turrets on a motorboat can provide some formidable firepower. While this is already very expensive on it's own, I want something even more devastating and expensive, basically a twin gun turret. It could use the already existing RPG as ammo, perhaps with the bullet drop physics of the cannonball. You could base it off of an real life gun such as the British BL 12in MkX naval gun, used in the HMS Dreadnought or you could make it fictional. Tek\Metal Tier would probably be the best choices, as probably only Alphas and Mega-Tribes would be able to make an essentially upgraded Rocket Turret. The shells it shoots can't be shot down by Auto Turrets, Hvy. Auto Turrets and Tek Turrets. To balance it it out it could be ludicrously expensive, although dedicated Alphas and Mega-Tribes could complete the requirements for the engram within a few days, so you could make a long reload timer, have a slow turning speed, or only a 180 degree turning radius. This might give alphas another tool for server domination but it also works against them. Some server Mega-tribes and Alphas can't be bothered to rule the seas with Motorboats clad in metal when they have Mosasaurs, Plesiosaurs, Sarcos etc. Ports, harbors, beach-side and forest edge bases could be vulnerable to amphibious naval fire from other Alphas and Mega-Tribes. A port base wipe could severely cripple a tribe's capacity to operate in the sea while securing the attacker's naval superiority. But for those tribes who are bothered to have Rafts and Motorboats probably have cannons and a couple Rocket Turrets at best, this could make those ships yield before this turret's awesome firepower.