The rail gun will be tested at sea in 2016, aboard one of the new Joint High Speed Vessels (JHSV). Then presuming that is successful, and I believe it will be, it will be deployed on the first US destroyer in 2018-2020.
THE US NAVY'S RAIL GUN
This will be game changing technology for close-in anti-air warfare, anti-ship warfare, and fire support for ground forces.