Quote Originally Posted by krsna View Post
The U.S. Army has just ordered another 1,095 Boomerang Sniper Detection Systems, and 2,195 vehicle installation kits. For decades, sniper detectors were theoretical darlings of military R&D geeks. But now, with lots of need, better technology and money to quickly buy several generations of a system, the devices are actually making themselves useful. Not all units have officers or troops who can make the most of sniper detection systems. But those that do, are hell on the local sniper population.

Most systems rely on the supersonic crack created by the bullet.
That supersonic crack is not being caused by subsonic bullets, which can be powerful nevertheless by being heavy.

Look at this, for example:
http://world.guns.ru/sniper/sn72-e.htm