The railgun projectile is only about 3 to 5 kg, very small, even smaller than the current shell used by most ship. The projectile used for the demo is about 3.2 kg, and the damage is little more than a Tomahawk can do. The warhead of a Tomahawk is about 450 kg. A lot cheaper. Nuclear sub is about the size of a destroyer, it doesn't take much to put nuclear power plant to a destroyer.