ERGM (pronounced ur-gum) was a United States Navy weapon system project. ERGM is an acronym for Extended Range Guided Munition. In a nutshell, the ERGM was intended to be a self-navigating gun shell to be fired from the U.S. Navy's 5"/62-caliber gun mount, the Mk. 45 Mod 4.
Where it came from
The ERGM has its roots in the Competent Munition studies performed at the Charles Stark Draper Laboratory in Cambridge, Massachusetts in the early to mid-nineties. Those studies, in turn, were brought about by the U.S. Navy's force structure and perceived mission of the same time period. The first Gulf War had shown the Navy that the perceived threats to the United States would come in the form of 'regional conflicts' - the 'yardstick' for U.S. forces was to be able to fight two 'Major Regional Contingencies' simultaneously. The real driver for the CM and ERGM, however, was a report issued in the early 1990s on the state of Naval Gunfire Support (NGS) available to the U.S. Around that time, the U.S. was busy decommissioning the battleships that it had resurrected in the early 1980s to anchor task groups against Soviet naval power. This was problematic, because the 16" guns aboard those ships were the best (and, really, only) option available to planners for NGS in aid of coastal or amphibious operations. With those guns going away, the only gun readily available to the fleet was the ubiquitious Mk. 45 Mod 2, a 5" gun fitted to the Arleigh Burke, Kidd and Spruance class destroyers as well as as the Ticonderoga, California and Virginia class cruisers.
The 5"/54, however, could only throw a shell a measly thirteen nautical miles, as opposed to the twenty-four that the 16" guns could reach, and its shell was a mere 70 or so pounds as opposed to the one ton bombs the battleships could lob. In order to do credible damage, the 5" shell needed to actually hit its target as opposed to merely come close, and it wasn't accurate enough to do that. Furthermore, the 13 mile range included the 5 to 10 mile 'safety zone' that the ships had to remain offshore in order to avoid making themselves vulnerable to light missiles launched from land; unlike the battleships, the more modern combatants carried little or no armor protection, and hits that the battleships wouldn't have noticed would have been devastating to the smaller ships; the experiences of the Atlantic Conveyor and HMS Sheffield in the Falklands War had demonstrated that to everybody concerned.
Extending the range of a gun shell was a readily-understood problem; base bleed, base burn and base boost were all available and varyingly effective. The problem was that increasing the range of an unguided ballistic weapon means a decrease in accuracy as the longer flight path means that all deviations from the aim point are correspondingly magnified. So, while the U.S. Navy wanted a shell that could reach out to 41 or 63 NM, they also wanted a shell which would hit its target so as to maximize the effectiveness of its (much) smaller payload than the 16" ammunition.
What it looked like
ERGM was the intended answer. It was a 5" projectile like the then-current shells used by the standard gun mount, but it was longer (5 feet, 1 inch as opposed to the three feet of a standard shell) and heavier (110 lbs vs. 70). It had a base-mounted rocket booster (base boost), and upon leaving the gun would extend canards for aerosurfaces in order to maneuver. The initial design of the ERGM contained 72 DPICM submunitions, which it would dispense in an area some 40-100 meters around the target. Later design changes in the project specified that it should include a variant with a unitary warhead, with appropriate changes in terminal attack profile.
The round was a navigating rather than seeking weapon. After launch, and during its ascent, it would utilize on-board power to run a GPS receiver. That GPS would be used to both navigate and continuously update an onboard Inertial Navigation System, so that in the event of GPS jamming or malfunction, the round would still be able to continue to its target point, albeit with slightly lower accuracy.
The round was designated the EX 171 (Experimental); had it been accepted and fielded, it would have likely become the Mk. 171.
What happened?
On March 24th, 2008 the U.S. Navy cut off further funding for the ERGM project, citing failures in recent tests. At the time the program, which had so far consumed approximately 695 million dollars, had been contracted out to the Raytheon corporation. A 2005 GAO report concluded that of the 20 'critical path technologies' in the ERGM, seven were 'not mature.' Raytheon announced that it was suspending work on the project, which employed 40 or so people in Arizona, and claimed that 'inconsistent funding' over the prior years had been a prime cause of the program's inability to perform.
As of this writing, it looks unlikely that ERGM will fly. A similar program for the Army, named 'Excalibur,' remains funded; Raytheon calls that a 'sister program' although it is being designed for different specifications, tolerances, and platforms.
Why?
ERGM was an ambitious project which suffered from many maladies familiar to students of military procurement. As the program progressed, it got bigger, heavier, more expensive, and more complicated. Initially, the Competent Munition studies envisioned not an entirely new round (which, incidentally, required the design and procurement of an entirely new 5" gun mount) but an applique upgrade via a smart fuze which could be screwed directly onto a 'normal' 5" gun round like a standard fuze before firing. The ERGM, on the other hand, had become a fully custom airframe and projectile; two feet longer, with mechanically extending canards rather than a bent ogive or fixed canard maneuvering surface. It dispensed 72 submuntions rather than relying on a standard bursting charge in the normal shell. Midway through the design phase, a requirement for a unitary warhead version was dropped onto it, necessitating a redesign.
While some ERGM prototypes did, in fact, perform as specified - leaving the gun, extending flight surfaces, acquiring GPS lock and navigating to the intended target - the round apparently never exhibited the sort of reliability that the Navy expected from a gun shell. Furthermore, the price tag for each individual shell kept ballooning. When the Competent Munition project was underway, the price for the fuze-based CM upgrade for 'regular' 5" shells already in inventory was on the order of $5,000 to $15,000 per shell, depending on the quantity manufactured. The ERGM, however, was up to $50,000 per round by the time of cancellation, and that price would likely have increased. When a proven missile system such as the Tactical Tomahawk or Standard Land Attack can deliver missiles for a couple of hundred thousand dollars each, firing $50,000 shells from a gun mount intended to deliver 10 to 20 per minute seems...wasteful.
In addition, the mission changed. The Nineties were the age of the Naval Force Posture entitled 'Forward...From The Sea', a plan for littoral warfare and the support of coastal operations including amphibious assaults. The post-9/11 world has seen two major U.S. combat operations in Iraq and Afghanistan, neither of which utilized significant Naval assets or contained ops which could have benefited from Naval Gunfire Support. Although (as locke baron points out) later hulls of the Burke, from DDG-81 to 102 and ships past that yet to be completed have the new gun, all isn't rosy. The new ships intended to carry the 5"/62 mount, the DD-21 Zumwalt, which may now be the DD-X or the DD-1000 Zumwalt depending on who you ask when, are still not online and their future is in doubt. Re-gunning the many existing DDG-51s with the original mount and other earlier ships has become less likely as upgrade funds have been cut over the intervening years. The fleet has been steadily retiring non-VLS-capable ships in order to maximize the number of missile-shooters available, and as they have succeeded they have diminished the urgency of having a high-tech gun support platform.
The Future
The basic principles laid out by the Competent Munition aren't going anywhere. One advantage CM-type gun munitions have over missiles and seeking munitions of all types is the potential cost; assuming one can manage to hold the requirements down to a reasonable level, a strong increment over 'standard' artillery should be achievable with much less cost than fully-seeking weapons or missile weapons. The Excalibur program may deliver a working CM shell to the Army for use in land-based artillery, which should be cheaper; the land based howitzer has a much less stringent requirement for its ammunition handling and firing profile, meaning the round won't be as complex or difficult to produce.
Furthermore, CM munitions do give a commander capabilities that simply aren't available presently using artillery. Since high-quality navigation and offset positioning devices are now widely available to ground forces, the availability of high-precision strike targeting information is available to a commander in a much more timely fashion, often in realtime, rather than relying on reconnaissance imagery or other rear-area support. With targeting information of this quality available, artillery with CM rounds would be able to offer much faster supporting fires, at much closer operating distance to friendly units, than traditional artillery - safe and fast enough, perhaps, to allow for automated call-for-fire using modern gun platforms like the Paladin.
Moving vehicles or uncertain target locations can be compensated for by using precisely-placed grid or box fires; when you are sure of your shells' ability to hit their designated aimpoint, efficient grid fires can be used to saturate 'probable' target locations with minimal expenditures of ammunition.
All in all, then, while ERGM may not have made it to the field, something like ERGM will no doubt appear in the near future. The various plans for new Navy ships contain something called LRAP for Long Range Attack Projectile, which is supposedly going to 'draw on' technologies from ERGM and the Excalibur. We'll see.
Disclosure: I worked on the Competent Munition project at Draper Lab in the mid-1990s.