There's a great thread on another forum, started by Ron Haney, who owns Battlefield Vegas (a tourist gun range).  They have various battle weapons, in various configurations, that get rented out.  Most are full auto and get a lot of use.

 

The thread is basically a field test of AR (amongst other weapons) durability in a controlled environment with documented round counts, maintenance schedules and inspections.  To me, it's a treasure trove of info on what works and lasts.

 

One of my take aways is the DI vs piston debate, specifically, what is "better".  Apparently, all piston systems cannot last like a DI system, with the exception of the HK416 and the Titan system.  Those two have decent life, but at the end of the the day, common sense says that more moving parts equals more chances of failure.  While theory is debatable, Ron Haney just provided the actual testing.

 

Here is a summary of the info gleaned from that thread:

 

HK MR223 has a very short barrel life.
Titan Defense (clone to the HK 416) is GTG, and just as good as the real 416 (but cost less).
AR15 (well manufactured) bolts will fail at around 30,000 rounds (around the bolt cam) at around which time the cam pins will fail.
PSA is very well made.....................
Colt 9mm carbine can be problematic.
SCAR-16 is very well made and a good alternative to the AR15.
The return springs on the operating rods of piston AR's need to be replaced around 5000 rounds.
The POF bolt cam pin is a NO-GO (I think I remember reading somewhere here, that the roller bearing comes off).
MP5 rock!

Magpul mags after hard use eventually crack on the back but they have excellent customer service.

Cheney. I've sent prospective sample parts to him to have them used in their guns because they do an excellent job of keeping records and putting tons of rounds through their guns.

Got a link?

Oops.  I thought he said "Haney" (but I was wearing hearing protectors).

 

http://www.ar15.com/forums/t_3_118/677135_High_round_count_AR_M4_s__over_100_000_rounds__and_how_they_have_handled_on_our_range.html

.  Apparently, all piston systems cannot last like a DI system, with the exception of the HK416 and the Titan system.  While theory is debatable, 

 

are you sure that's debatable?  i thought it was pretty much as well known as the fact that eotech's suck.  sure, lots of people still think putting a piston and/or eotech on an AR would be "better" but they're wrong.

Im feeling rather vindicated these days