I had a visit from my BIL & he brought several rifles to sight in. One AR defied our attempts. From the bench at 50 yds. the best either of us could do was 5" - 6" groups. My eyes are no longer iron-sight friendly but they aren't THAT bad. We finally gave up & as I was gathering up the brass I noticed rings or steps in the shoulder of the case. This was with 2 different loads, PMC 55 gr. & 62 gr. green tip. Just to be sure it wasn't an ammo problem I fired both loads thru one of my AR's & the brass looked normal. I'm guessing there was some problem with the reamer when the chamber was cut & it'll take a new barrel to fix it. Any insight anyone can offer would be appreciated.