The justification test for the use of deadly force is, was it reasonable, and was it necessary? Current LE training doctrine is to avoid lethal force until it becomes unavoidable, and when shooting, shoot until the threat is gone. There is no set number of rounds that are reasonable and necessary-in all use of force cases it depends upon what was reasonable and necessary under the circumstances. There are cases where it took a lot of rounds to bring an assailant down, but usually no more than 3 shots are fired. It just depends on a whole lot of factors. If the subject is on the ground but still presents a threat, more shots may be justified. In the case cited where the officer fired multiple rounds into the downed subject, the subject still had his rifle. What is reasonable in the heat of battle may be very different from what is reasonable in the seconds after the subject is down. There is a line that you do not want to cross that takes your actions from self-defense to Coup de Grâce execution of a wounded opponent. A case in point is the Oklahoma pharmacist who was convicted of murder and sentenced to life in prison for shooting an armed robber 5 more times after he was wounded and down. The irony of that case is had he killed the guy with his first shot he would have been a hero.