Navy railguns will launch 23-pound projectiles more than 100 miles at Mach 7 (5,300 mph)

Prototype of Navy's high velocity rail gun under development

From NextBigFuture.com, Feb 9

The US Navy has a roadmap to align electrical power system developments with warfighter needs and enable capability based budgeting. It is meant to be a living document, updated biannually, that invites innovation and guides investment by DOD, government, industry, and academia to achieve synergistic advances in naval power systems.
Recommendations have been provided based on available information, engineering judgment, and projected requirements. The historic timelines for major component and large system development such as the gas turbine engine and IPS can take up to about 20 years to transition to the fleet whereas smaller subsystems such as the LHD 8 hybrid electric drive can take up to 8 years. During the same length of time, the Navy 30 year Shipbuilding Plan changes, ship programs are initiated and terminated and threats to our security change constantly. This TDR proposes multiple paths to continue providing targets in the face of uncertainty.
This is a 95 page document. Naval Power Systems Technology Development Roadmap PMS 320 by the Electric Ships Office that is directing the Future of Ships Power. Advanced sensors, railguns and lasers will need a lot more power but that power will need to be compact and light. 
The Navy has just installed a first prototype railgun on a navy catamaran. The Navy has installed their prototype railgun on a 1500 ton military catamaran called the joint high speed vessel aka JHSV. Lance M. Bacon reports this development at the Navy Times and Sam LaGrone at UNSI. The JHSV has 600 tons of excess payload capacity, but Zumwalt Navy destroyers have 20 tons of excess payload capacity. The railgun will need to be made more compact to be installed on the destroyers. The Destroyers at 14500 tons are bigger than the JHSV. The Destroyers are already filled with other systems.
The railgun also uses 34 mega joules of power to launch a 23-pound projectile to distances greater than 100 miles at speeds topping Mach 7 (better than 5,300 mph). While Zumwalt-class destroyers will generate roughly 78 mega joules (twice the power the railgun needs), most destroyers have in reserve less than one-third of the power the railgun needs. And there are a whole bunch of limitations on what can be done inside the ship to add power generators. Developers will house power generators in con-ex boxes during the JHSV demonstration, but a permanent power solution will be needed before the railgun lands in the fleet.
The target is 2018 for installation of a combat railgun on a US Navy destroyer.