Ballistic Radio has been running their Knight’s SR-15 pretty hard. How hard? They’ve so far fired 9,500 rounds, without any cleaning or weapon-induced malfunctions. The picture at the title of this article says a lot, but there’s a video as well.
Realistically, there are very few weapons as reliable as a well-made, well-maintained AR-15. Ballistic Radio’s goal is to reach 20,000 rounds. Best of luck to them!