Could call it a macro cannon or such I guess. That might be too much weapon variety creep though.
fakeedit: Notably in 40k a macrocannon is like a giant fusion bomb gun, which might actually be more fun than just a giant railgun as I was picturing originally. you could build engine-less missiles to shoot out of it like bombs and use that as ammunition, but otherwise it shoots like a beam. that might be a fun concept as a sort of bombardment siege gun or for close range anti warship duty (assuming you were fairly sure you werent going to miss with too many of your precious nukes)