How does Law Enforcement manage to get by with M&P15s?

[markm]

I'm seeing an interesting assortment of statements on the subject, but the consensus seems to be that LEOs don't shoot enough for any of the parts on an M&P15 to break, which begs the question, how many rounds would an M&P15 fire before parts would begin to break, and how close is it to the average round count which would cause parts to break on a Colt 6920?

I've read several general observation articles/post by instructors who've outlined their observations on ARs having failures. Can't remember M&P being mentioned, but indeed the Colts/BCMs tend to be the most trouble free. You have other good guns out there like DD and LMT, but I don't know how commonly those are found in training... opposed to a 6920 which at least a few are probably in every class.