A lot, actually. I wouldn't want to go on a raid with a bunch of untrained gun owners, that's for sure. If you put your average untrained firearm owner in the same high stress situations, the outcome would be drastically worse.
Comparing what police do daily to what aveage joe gun owner does daily doesn't hold water. Unfortunately mishaps happen to both.
You do realize Untrained Gun Owners across the World have defeated some of the best Trained Military Units out there?!
From personnel experience of having shot in High Stress Situations with LEAs...
You can keep the Majority of LEOs far, far, far away from me when the High Stress part hits the Rotating part...