If you are getting a first shot fly, it's a chambering issue called "bolt over run". There's too much clearance between the barrel face and the lug seats in the barrel extension. The cure in ARs is to get a new barrel.
You can get a feel for the problem by inserting the bolt into the barrel and twisting it to lock. There shouldn't be more than about 3 thou of forward-backward play in the bolt.
Last edited by BarryC; 10/18/14.
