The police department where I live recently ordered conversion kits for all their AR-15s. By going to the conversion, the officers will be able to get a massive amount of trigger time for a fraction of the cost due to the price difference in ammunition.

CheaperThanDirt.com (standard disclaimer, I have no affiliation) shows .223 Remington for 81 cents a round, while .22 LR is 7.2 cents a round. So, you can shoot 10 rounds for the price of one.

I've not talked to any officers who've fire them, but they were excited about the prospect of being able to shoot hundreds and hundreds of rounds at the range. I'll check with a couple guys tomorrow to find out their thoughts after shooting with the conversion kits.