FBI Canine Killsquad/Occasional Hostage Rescue used em for a while. I remember reading a thread about how one of the major gripes was that the stock bolt/springs were too heavy to cycle cheap range ammo for training, and using the lighter springs would tear the weapon apart with regular pressure ammo. So they would switch them back and forth for range time/duty. Armorer's nightmare. Pic sorta related since I can't find the HRT photo I had saved.