I bought a Sabre 20" upper from PK a couple years back before going to Iraq but never really shot it, when I came home I had a bunch of other guns that I was into so it was pretty much a safe queen. When I finally took it out to the range a few months ago I tried to zero it with my ACOG and surplus M855 ammunition at 100m and wasn't able to hit the paper. I brought it in to 25m to do an alternate zero and I was getting two good hits followed by a third keyholed off somewhere random. I packed up out of frustration and came back a couple weeks later using Federal XM193 to see if it was just the ammo (the M855 shot fine through both mine and my cousin's CMMG M4geries). Groups were a little more consistent but when the barrel heated up they opened up to unacceptable sizes and again had issues keyholing. Tried with Prvi M855, same thing. Not sure what would cause it, but I was wondering what the warranty on Sabre products is now that they are gone.