What type of sights? If you are shooting iron sights and it is a sunny, and the sun behind you and high, it can make your group off to one side or another. Say the sun is on your left side and high behind you. The left side of the front sight is illuminated. You will put the bright side in the center and it will cause you to shoot to the side. People shooting in competition will put "sight black" on the front sight so it is a even black sight. You would not think it would make a difference but it does. Since you were shooting a good group I would look at this or perhaps a consistent jerk or flinch.