I had a buddy win a DPMS SASS. I believe the barrel came loose or barrel (ETA: typo) nut loosened and developed wobble at about 300-400 rounds?
I can find specifics, but it's off hand and about 4-5 years ago.
I'd go SCAR 17. Light weight (good for carrying), folding stock (great for a pack), consistently very accurate and reliable. They're great guns, I was considering a SCAR 16 for sometime. I just eventually came to the conclusion that it didn't offer a ton my BCM AR's didn't as far as reliability, etc.
The downsides are 16" is getting to be short at 500 yards and SCARs beat up optics; so factor that into your decision.
I'd recommend a 2.5-10x scope for 500. Maybe a 1-6 at least?