I have a Bushmaster ORC in .308 with 1:10 twist. I tried to sight in using mil issue 7.62, 175 gr boat tail, and it seemed like the scope was losing center. Some high, some left, some right. After a little rest, I tried some 146gr ball, and the thing grouped around 1" at 100 yards (but I was getting sweat running down into my glasses). I have seen where some ammo will do different things, but this is something that has me baffled. Any ideas?