By the sounds of it, this thing needs a lot of power to fire just one slug. I don't know how much wattage the 32 megajoule blast converts into, but that energy burst seems to last a fraction of a second so, probably, this cannon eats up hundreds of megawatts on a single shot.
If that's the case, high power lasers could be a much more efficient method of defense (more so because they already exist and are practically combat-ready now).
Which means that naval artillery is pretty much the only military application for a rail gun ... dunno, doesn't really seem that useful when compared to other weapon systems like ship to ship missiles and naval aircraft.