According to researchers at the U.S. Army Armament Research, Development and Engineering Center, a pyrotechnic material is ignited when a rifle is fired, which in turn ignites a reactive material designed to transform a .50 caliber bullet into an "aerodynamically unstable object." In effect, the burn time dictates how far the bullet will travel before it loses its aerodynamic shape and the velocity falls.
Researchers Brian Kim, Mark Minisi, and Stephen McFarlane filed for the patent on May 7, 2013 and was approved on September 1st, 2015. Although it's generally agreed that the concept would reduce collateral damage in cities and could span "various calibers of small arms munitions," there is currently no funding for the project. It's hoped that the idea will "resurface as the constant need to provide greater technology for the warfighter increases" -- whatever that means.