Snipers are extremely dangerous. Locating and neutralizing them is a need of both the law enforcement and military communities. The main means of gunfire detection today, i.e., the human ear and eye, are highly inaccurate. The JPSG intends to develop and demonstrate an affordable sniper detection system that can detect and locate a sniper, to within a 10 foot x 10 foot box, in urban, as well as rural environments, to ranges in excess of half a mile. Urban environments are challenging because manmade structures cause echoes, complicating detection by acoustic means, and hide visual cues, i.e., muzzle flashes. Another major technical challenge is motion compensation for sniper detection systems mounted on moving vehicles or worn by soldiers or policemen. While more challenging technically, mobile systems offer inherently greater flexibility than fixed site systems.
This effort is developing systems that can be: 1) carried by and put in place by hand; 2) worn; and 3) mounted on vehicles. Technologies being explored employ acoustic, IR, integrated IR-acoustic, and integrated IR-laser sensors. The picture is of man portable acoustic systems that were demonstrated at Camp Pendelton, CA, in May, 1996. They are shown at the Pendelton test site.
Six prototypes of the best performing acoustic systems, from the Camp Pendelton demonstration, will be fabricated and provided to the military, potentially for use in ongoing operations such as Bosnia.
In July, 1996, a man portable, integrated IR-acoustic system will be demonstrated at Camp Pendelton. This will be followed in February, 1997, by demonstration of an integrated IR-laser system. This demonstration will also be at Camp Pendelton. This system tracks a bullet in flight from the heat caused by friction as the bullet passes through the air. The bullet's track is used to determine its three dimensional trajectory. From this information the system will determine the exact location of the sniper. This system will locate snipers even whey they employ advanced tactics and special devices such as silencers and flash suppressers to conceal their location.
Demonstrations of sniper detection systems that are worn and that are mounted on vehicles are scheduled to be completed by the summer of 1997.

This is from a piece at the DOD/ATCD Counter Sniper page.

Of the range of emerging sniper detection capabilities available in May 1996, four were selected as ACTD participants. The four selected systems were perceived to have already demonstrated some significant capabilities, were part of funded DoD programs, and required little or no additional engineering to bring them to a state suitable for fielding. Each of these systems represents a different technical approach to the problem and each has its strengths and weaknesses. A brief summary of each is given below.
Compact acoustic shockwave detector: The selected system of this type is called the Bullet Direction Indicator (BDI). Five units were procured. The phenomenology exploited is the sound of the shock wave generated by a supersonic bullet. Typically the bullet must pass near the sensor to be detected. The system indicates the direction to the shooter with fair accuracy and has a complete 360 degree field of regard. This system is simple and low cost. The Army Research Laboratory (ARL) previously purchased these five units as non developmental items for evaluation in conjunction with the DBBL.
Program Status: This system was evaluated by ARL and found not to be technically mature enough to deliver to the DBBL for user evaluations. A number of manufacturing problems were identified and corrected but the system did not perform well enough to warrant further evaluations. Work on this system is continuing via a cooperative program between ARL and the U.K. Ministry of Defense.

Distributed acoustic muzzle blast and shockwave detector: The selected system of this type is called Bullet Ears. Five units were procured. The phenomenology exploited is the sound of the gun firing and/or the sound of the shock wave generated by a supersonic bullet. This sensor system consists of two sensor elements, separated by a distance ranging from a few meters up to 100 meters. The system can locate the position of the shooter in three dimensions and has a complete 360 degree field of regard extending to the range of the threat weapon. This system is relatively low cost. DARPA provided this system as the most promising of the ongoing Counter Sniper Program.
Program Status: This system performed well at technical evaluations at Camp Pendleton in May 1996. Five units were built and tested at the DBBL in September 1996. Evaluations were favorable and the DBBL currently has the five units available for deployment. DARPA is continuing development of this into a helmet mounted system.

Widely distributed acoustic triangulation system: The selected system of this type is called TAG-IT. One unit was procured. The phenomenology exploited is the sound of the muzzle blast. Each sensor element consists of a single microphone and a radio. When a shot is fired, a subset of the sensor elements detects and identifies the muzzle blast and reports the time of detection to a central unit via radios. The central display unit shows the location of the shooter. The accuracy is fair to good. The spacing between sensor elements is a few hundred meters. The individual sensor elements are low cost. The TAG-IT system is based on the SECURES (System for the Effective Control of Urban Environment Security) technology, which was developed by DARPA for law enforcement applications.
Program Status: Technical evaluations were completed in April and May 1996 at Camp Pendleton. User evaluations were completed at the DBBL in May 1996. This system was subsequently deployed during the 1996 Olympic Games, a portion of which was held at Ft. Benning. Additional tests were also successfully completed in Dallas Texas in November and December 1996.

IR muzzle flash detector: The selected system of this type is called Viper. Four units were procured. The phenomenology exploited is the heat generated in the IR spectrum (3-5 microns) by the gun firing. The accuracy and detection time is excellent. The sensors have a limited field of regard (30x22 degrees) but can detect threat weapons beyond the effective range of the weapon. The system directs an observer to the threat location using computer generated symbols incorporated in the observer's binoculars. Viper was developed by the Naval Research Laboratory (NRL) as part of an ongoing 6.2 research effort.
Program Status: Technical evaluations of the Viper were completed in June 1996. Operational tests of four units were completed in August 1996 at Ft. Benning. While the equipment was somewhat bulky, the system performed well operationally.