The U.S. Navy recently completed four successful tests of its new 155mm AGS (Advanced Gun System) cannon. The LRLAP (Long Range Land Attack Projectile) GPS guided shell performed as expected and was fired out to 83 kilometers. It was only two years ago, after six years of development, that the first successful test firing of the 155mm AGS took place. The navy would like to replace existing 127mm guns on some warships with the AGS, if only because the 127mm gun has a much shorter range (24 kilometers) and uses only unguided ammo.
Designed for use on the new DDG 1000 ("Zumwalt") destroyers, the AGS fires GPS guided shells up to 190 kilometers. So far test firing has only been to about half the planned range. 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 was originally supposed to enter service in 2015. That has now been delayed at least three years.
The U.S. Army has a similar round, the Excalibur, which entered service six 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 wanted 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. Some naval officers have urged the adoption of the army 227mm MLRS rocket but there’s too much support for AGS for that to happen, at least not yet. Meanwhile, Italy has put into service a GPS guided 127mm shell (Volcano) that has a 100 kilometer range and works. The Italian 127/64mm gun is a bit longer than the U.S. 127mm/54mm models, but that would not reduce range of the rocket assisted Volcano shell by much. While there is resistance to buying foreign weapons for U.S. ships, there have been several exceptions in the past few decades and Volcano may be another one. The AGS is really too big and power hungry to fit on existing American destroyers and cruisers.
The Excalibur technology could be adapted for use on the 127mm gun most American destroyers carry, as could an even cheaper (and less accurate) technology that uses a larger fuze (the device that is screwed into the front of the shell to handle detonation) containing the GPS receiver and some movable fins to guide the shell. But the navy guided shell effort has burned up so much cash, and failed so many times, that growing budget cuts may just cause the effort to be halted, for now. Or at least until the Chinese reveal they are working on a similar shell for their warships.