Most of us know that bullets don't fly in a straight line. However, they don't
exactly fly in a symmetrical ark either. This video shows the direction that a bullet is pointing (yaw & pitch) as it flies down range. You can see
how long it takes a bullet to stabilize in flight. As the bullet gets farther away, its flight settles down and the bullet path becomes more consistent.
This explains how your rifle might well be able to shoot one MOA groups at 100 yards, and then shoot half MOA groups at 600 yards using the exact
same ammunition. This is quite common, because bullet flight will usually stabilize at 200 yards or beyond. Anytime a bullet enters (or exits) the
speed of sound, its path becomes unstable and accuracy will suffer.