I bought a 10" .223 barrel about a year ago that had iron sights, and I had the same problem.  It turned out that the screws holding the sight base to the barrel had come loose, so the sight was shifting around with the recoil.  I found this out because I thought it was a faulty sight, and found the loose screws when I removed the drift pin holding the sight armature, exposing the screws, when I started disassembly to put a scope base on.  I tightened the screws down, reassembled the sight, and it held zero just fine.