Video of it in use [1:00] (no audio): https://youtu.be/YoOaJclkSZg
The Department of Defense’s research agency, DARPA, releasedfootage on Monday showing the bullet, after completing a prototype in February, as part of DARPA’s Extreme Accuracy Tasked Ordnance (EXACTO) program. Footage posted to DARPA’s official YouTube channel shows live-fire tests by an expert sniper and a first-time sniper. In both cases, the bullet course corrects in midair, speeding toward a target even if it’s not centered in the crosshairs. An optical guidance system enables the bullet to compensate for weather, wind and other factors that might push it off course.
https://www.thefirearmblog.com/blog/2015/04/30/darpas-exacto-guided-bullet-makes-progress/
My money is on them being cost-prohibitive to do at scale
If you do things right, you just need one. Maybe launched from a tiny drone loitering out of sight.