The 54-calibre, 13cm naval artillery shells fired by Arleigh Burke-class destroyers are brutally efficient at destroying surface ships, aircraft and land targets. But they're little more than pea shooters compared to the 15cm rocket-propelled, GPS-guided shells shot by the new Zumwalt-class' Advanced Guns System.
The Advanced Gun System is the next generation of naval artillery designed by BAE Systems for use aboard the new Zumwalts when they launch in 2015. A pair of 155mm calibre AGS will constitute the new destroyer's primary guns and are capable of firing up to 10 rounds of precision munitions per minute through their water-cooled barrels. What's more, the system is entirely electric, although its massive 800kW consumption severely limits which ships in the US fleet can incorporate them -- at this point, only the three upcoming Zumwalts have the power supplies and space needed to support them but given the time and money BAE's sunk into their R&D, it's very likely they'll be adapted to future warships.
The destroyer's automated Intra-Ship Rearmament System (AIRS) handles reloading the guns using a series of 2700kg pallets, each holding eight propelling charges and eight 104kg Long Range Land Attack Projectile (LRLAP) shells, for a total magazine of 304 shots. Interestingly, while the AGS fires a 155mm projectile (which is pretty much standard for US field artillery) it can't actually fire normal Howitzer rounds, only LRAPs.
An LRAP is basically an 223cm long rocket-propelled artillery shell with GPS-guidance. With a 11kg blast-frag warhead, the LRAP is roughly equivalent to the M795 artillery shells used in modern M119 Howitzers, but it travels much further and much more accurately. Current naval 13cm guns have a range of roughly 13 nautical miles, the new AGS can heave an LRAP more than 59nm. In fact, during flight testing, the munition accurately hit targets up to 100nm, falling within a radius of less than 50m. Future iterations could even employ IR seeker heads -- you know, to better shoot down planes and such. [Wikipedia, Wikipedia, Defense Industry Daily, Lockheed Martin]