This phenomenon occurred with my PTR but I'm posting it as it might be a common issue. Anyway I bought a new PTR GI. I shoot some 2.5 inch 100 yard groups. The rifle was shipped with a BW hand guard which is worn loose enough at the receiver end to move almost an 1/8" up and down. Side to side its 1/32 or so. I installed a newish BW 1997 hand guard which fit so tight I had to tamp in the pin with a piece of wood. My groups opened up to about 5". I relieved some of the plastic and aluminum at the muzzle end to clear the barrel all around. still no better so I sanded a bit at the receiver end to shorten it over all so I could put the pin in with finger pressure although hand guard still fits snugly. Now its groups tightened up a little but as I shot the center of groups move about 5" across the target. So I put the beat up loose hand guard back on and presto I am centered with a nice group aligned right down the middle of the bulls eye.