September 30, 2011:
Recently, after six years of development, the U.S. Navy successfully test fired its new 155mm AGS (Advanced Gun System) cannon. Designed for use on the new DDG 1000 ("Zumwalt") destroyers, the AGS fires GPS guided shells up to 190 kilometers. The recent test firing of two shells only went out to 81 kilometers. The GPS guidance enables the shells to land inside a 50 meter (155 foot) circle. The AGS shells carry 11 kg (24 pounds) of explosives. The AGS uses a water cooled barrel, so that it can fire ten rounds a minute for extended periods. Each AGS carries 335 rounds of ammo, which is loaded and fired automatically. The AGS shell is expected to enter service in three years.
The U.S. Army has a similar round, the Excalibur, which entered service four years ago. Excalibur has a max range of 50 kilometers and will land within a 20 meter (62 foot) circle. In practice, Excalibur will land within a few meters of where it's aimed. Each Excalibur shell carries 9 kg (20 pounds) of explosives. The AGS shell has a longer range because it is fired from a longer barrel using a more powerful propellant charge. AGS rounds are also capable of the same accuracy as Excalibur, but it depends on the quality of the GPS signal in the area.
The new AGS round replaced an earlier project, that ended in 2005, after twelve years of effort, and two billion dollars, to develop a GPS guided round for a five inch (127mm) naval gun. This ERGM (extended range guided munition) system never worked reliably. So the navy went looking for another solution. Taking note of the success of the 155mm Excalibur, the navy ended up using that technology for its AGS. The navy wants to use AGS on new warship designs, in order to get more effectiveness out of the limited amount of ammo a ship can carry. Accuracy is the key. A "dumb" (unguided) artillery shell will land with 75 meters (or more, depending on range) of the aiming point, while the laser guided Copperhead (an older army 155mm design that was too expensive) would land within a meter or two. GPS guided shells will land within 3-25 meters of the aiming point.
Excalibur has proved very popular with army troops, but with so many other guided weapons available (especially the 227mm GPS guided rocket), not many are used. In Afghanistan, 5-10 Excalibur shells are fired a week. For this reason, AGS may never be heavily used for supporting troops ashore. Adding a terminal guidance system to the AGS shell would make it suitable to attacking other ships.