The other is the DD(X), a next generation 12,000 ton destroyer (v. about 8,300-10,000 tons for current destroyers and cruisers) most notable for the fact that it will require about a third of the crew of current destroyers ("The DD(X) would be staffed by about 114 sailors instead of the 360 required on an existing DDG-51 destroyer."), is also supposed to have a somewhat smaller radar profile, and will have a long range naval gun that will allow it to fill a gap in fire support for soliders and Marines in coastal areas created when the Navy decommissioned the last of its battleships. Initial plans to mount an electromagnetic rail gun on the ship have been abandoned and replaced by a more conventional gun:
The AGS gun system will be capable of firing up to 12 rounds per minute from an automated magazine storing as many as 750 rounds (but usually closer to 350). The AGS ammunition is equivalent to the USMC M198 155mm Howitzer in firepower, and also includes development of a 155mm version of the Long Range Land Attack Projectile (LRLAP), a GPS/INS guided munition capable of hitting targets accurately up to a distance of 100 nautical miles. Future variants may also incorporate seeker heads, and efforts are underway to achieve as much commonality as possible with U.S. Army 155mm projectiles.
The AGS gun system still requires a great deal of electrical power, even though it is not a rail gun, so it may be impossible to retrofit onto most older ships. It is also too small to be very effective against other warships, although it might be effective against a group of enemy infantry or a tank or an oil rig or smaller boats.
The largest naval gun currently in use in the Navy is a 5" gun (127mm) which is found on cruisers and destroyers and has a 13 mile range (up to 60 miles with advanced munitions). Frigates have a 3" gun (76mm) with a roughly 10 mile range. Battleships used 16" guns (with rounds about twenty times as large as those of a 5" gun) with a 20 mile range (although an advance munition likely could have gone farther).
Existing frigates, destroyers and cruisers rely on their guided missiles, virtually to the exclusion of their naval guns, to engage hostile ships and distant targets on land, but those guided missiles are very expensive compared to the ammunition in the naval gun, and don't have a terribly high rate of fire, so they are ill suited to supporting Marines on the beach and other coastal ground troops.
The DD(X) program has been dramatically cut, from an initial planned run of 32 to just 8 ships (or as few as 5 if they get more expensive). And, their cost has ballooned to an estimated $3 billion per ship. For comparison purposes, the Nimitz class aircraft carriers which are currently the largest and most expensive ships in the Navy cost just $2.5 billion each (the George H.W. Bush, which is the last ship in that class, is scheduled to be delivered in 2008) (although new aircraft carriers are expected to cost about $14 billion each), and new Virginia class nuclear submarines cost about $2.1 billion each.
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