The US military and CIA have used "drone strikes" heavily in the war in Afghanistan and the hunt for members of Al Qaeda. They've fired Hellfire missiles from the long-range Predator, Reaper, and Global Hawk unmanned aircraft, killing over 3,000 people so far with such attacks. But at 100 pounds, the Hellfire weighs almost as much as some small drones, and it costs tens of thousands of dollars. Add that to the cost of flying the big drones that carry them, and it makes for a very expensive way to kill someone.

But the economics of "precision-strike" drone warfare may change very soon. The US Army and General Dynamics announced today that they have successfully tested a weapon that could turn small unmanned aircraft into small-scale automated precision-attack bombers. General Dynamics' Ordnance and Tactical Systems unit and the Army Armament Research and Development Engineering Center successfully demonstrated they could shell targets from a small drone armed with GPS-guided 81-millimeter mortar rounds.

Originally designed to be fired from mortar tubes on the ground, GPS-guided mortar rounds started to be deployed to US troops in Afghanistan last year. The guidance is part of a fuse attached to an otherwise-conventional mortar round that controls the stabilization fins on the shell. The system tested by ARDEC and General Dynamics (called the Air Drop Mortar) turns GPS-guided mortar rounds into "smart" bombs, programming them with GPS coordinates through a specially designed rack on the drone.

In a series of three tests using a TigerShark drone—a 200-pound drone with a 17-foot wingspan capable of taking off and landing on its own—the Air Drop Mortar (ADM) dropped the 10-pound test shells from about 7,000 feet. They fell within seven meters of the designated target in every test. The blast radius of a high-explosive 81-millimeter mortar shell is about 35 meters.

There are a number of military advantages to dropping mortar rounds from drones. Drones obviously have a longer reach than artillery, and they can linger over an area for long periods and attack without warning (the TigerShark drone can stay in the air for up to eight hours). And compared to the missiles typically carried by DoD and CIA drones, mortar rounds are dirt-cheap.

But there are other potential uses for the ADM. Mortar shells can also carry non-lethal contents, such as smoke and gas, so the potential applications of mortar-armed drones could easily extend into law enforcement and homeland security. (Yes, small drones could be launched from a distance to drop shells to disperse crowds.) Law enforcement agencies are already looking at the potential of using drones with non-lethal weapons. The sheriff's department of Montgomery County, Texas has expressed interest in arming its Shadowhawk helicopter drones with tear gas grenades, rubber bullets, and tasers.

Of course, the accuracy of the ADM, and of the drones that carry the system, could be put at risk by GPS jamming technology. The Iranian forces allegedly used such tactics to capture a CIA RQ-170 Sentinel stealth drone last year. And the general security of drone systems is already in question. The video feed from many US drones is unencrypted, and has been intercepted in the past by enemies on the ground using satellite communications monitoring software.