I am not a cop and never have been. I know a few very well, one of which had a lot of firearm training in the Marines, and this is basically what they told me:
Most cops are not well trained with firearms because they don't shoot enough and when they do shoot it's often simply to qualify (once or twice a year). Many departments don't have much money to spend on ammunition for training. Obviously there are exceptions, such as those who did a lot of firearm training in the military and some SWAT personnel. Cops are well trained drivers, certainly not Richard Petty in his prime caliber, but much better than the average person.
I can tell you from experience that those with a combat MOS in the Army do A LOT of firearm training and I don't recall ever having a shortage of ammunition to shoot for training purposes. On the other hand, many with a non-combat MOS talked as if they were lucky if they got to a range two days a year, and they usually didn't get to shoot at moving targets or shoot at night, and they never get to train in an urban environment.
I strayed a little from the topic. Oh well.