OP I think is referring to bullet Yaw as the bullet exits the muzzle. It does take time for some bullets to "go to sleep".
Aberdeen Proving grounds takes pictures of bullets as the exit the muzzle, clearly demonstrating "Yaw". Particular VLD designs take longer to spin on their own axis, while a round nose bullet runs on it's axis very soon after exiting the muzzle.
While a bullet is in Yaw, the BC is less, or course.
