I have an AR15 with a Burris XTR-14. I have it sighted in at 200 yards. When I try to shoot out at 500 yards, it seems to be all over the place(8 inch group). But I can shoot a 2" group at 200 yards and make the same hole bigger at 100 yards. I was shooting 62gr. M855 and had the rifle rested on...