The reduced BC has to do with the bullet's epicyclic motion. There are two arms to this motion- the nutation arm and the precession arm. The nutation arm is mostly damped out by 100 yards, but the precession arm typically remains constant, or it may decrease or increase as the bullet flies down range, depending on bullet mass and aerodynamic properties, and twist rate. It is this precession arm in which we see less stabilized projectiles exhibit greater yaw and pitch, regardless of distance. When gyroscopic stability is insufficient to overcome dynamic stability, the bullet settles into a greater limit cycle yaw angle, and BC suffers.
Typically for every 0.1 factor decrease between 1.0 and 1.5 using the Miller stability formula, you can expect about a 3% decrease in BC value.
