PDC's use "conventional" ammo, but the main weapons of the ships are indeed Railguns (other than the rockets they shoot at eachother).
Edit: Then again, now that I think about it, there is recoil when firing the main weapon of the Roci in the books. Maybe they haven't found the button for it yet in the show.
Edit: Then again, now that I think about it, there is recoil when firing the main weapon of the Roci in the books. Maybe they haven't found the button for it yet in the show.