I am not a law enforcement officer, but I group around them and my dad was a law enforcement officer who was a field training officer as well as being in charge of the range, range requirements, and any training that came along. I may have had access to materials that the public doesn't normally see in that regard.
In instances where an officer (or civilian actually) is firing under duress (as defined emotionally, not necessarily physically) it is extremely common that once one round is fired the gun is fired until empty. It has also been observed that when in a group if one officer fires, it is likely that the entire group will shoot until the gun is empty. It is also pretty usual that most of the bullets miss at any range, this is because under stress the first thing you lose is fine motor control. This probably also accounts for why it is so difficult to stop once the first bullet is shot.
The OK Corral is a good case study. A massive hail of bullets, very few of which hit anything, and this was skilled law enforcement against skilled outlaw.
