I selected a battleship hull from the hull templates and then outfitted it with three enhanced railguns, promethium drive, advanced shield generator, advanced point defense, 2x hull plating, foundry and a slipstream accelerator. It does have the battleship icon right beside it in the list. So, I iteratively removed one module after another until there was just one railgun on it. Still no luck. So, I went and started from scratch, same hull, just one railgun on it. Same result. So, I thought maybe I am doing something wrong and just went to design from scratch, without cosmetic pieces, I grabbed one of these Slyne sphere models and put a railgun on it, same result.