Unless those civilians have been trained under live-fire circumstances, there is a far greater probability that they would add to the toll of bystander injuries

Unless those civilians have been trained under live-fire circumstances, there is a far greater probability that they would add to the toll of bystander injuries than that they would quickly stop the original threat. It's not life on the target range; flinching, adrenalin tremors and tunnel vision are real, and a miss (or a through-and-through) doesn't always go safely into a sand berm. And what about the other armed individuals whose situational awareness is formed when they see two or three shooters rather than the original one?