I am pretty sure any one that says the AR will gunk up and not work (personally, I have never known someone to say this) must be using very old crappy ammo and haven't cleaned their rifle in about 1000 rounds. Just use decent ammo. And I mean decent, not the most expensive, just decent ammo you can get at Walmart. I am pretty sure ammo such as Federal or Sellier and Bellot can go 1000 rounds no problem.
Personally, I shoot 100 rounds in my AR at a time and the rifle is still pretty clean and thats with the cheapest brass ammo I can find.