In Dr. Mann's book, Part II, starting on page 268:
There's excellent graphs and pictures showing how unbalanced bullets spiral around the trajectory axis. Also shows his cards aligned on his test range so the bullet paths could be graphed. Read the whole book; interesting stuff indeed. Not too shabby at all considering Dr. Fredrick Mann was a surgeon of great esteem who just liked to shoot rifles and learn why their bullets went where they did. The copy I have contains notes written by Harry Pope; famous barrel maker of the early 1900's.
As I recall a chat with Sierra's Bob Hayden some years ago that their time of flights tests past 100 yards show bullets starting through the test screens had more consistant and higher BC values as those tested in the first 100 yards. They attributed that to bullets' stabilizing better flying point on and their long axis perfectly parallel to the trajectory axis compared to those at shorter ranges didn't fly so well stabilized. But their tests also show that even the best bullets' tiny unbalance still cause a BC spread of 1% or thereabouts.