I'll have a report on this soon, as well as a more extensive report on the .408.
Basically, BC decay or degradation is caused by the bullet overspinning which causes precession and yaw. Precession and yaw result in added drag with time. The exact rate of decay is a function of the load and and the particular barrel from which the bullet has been shot. Overspinning is caused by axial drag being less than horizontal drag in most bullets. Only bullets designed with the axial and horizontal drag balanced, the Balanced Flight Theory, can exhibit constant BC flight.
So how does one know if a certain bullet is properly balanced? I would make the assumption that most VLD bullets are better balanced than conventional designs but this does not mean that a given VLD design is properly balanced.
Sierra lists varying BC figures for their bullets at different velocities. Is this a function of precession and BC decay or do bullets really exhibit changin BC values at different velocities?
Arrrrghhh! My brain is starting to hurt!
I ran into a very strange phenomenon the other day. As expected, at 1900 yards the round hit quite a bit low as predicted by the ballistics computer.
However, on another day, at a much different temperature, the round hit about 8 MOA high at 1500 yds. Same exact round, load and rifle. Wierd!
I have only been out once so have not noticed if this is simply a wierd set of circumstances or an indication of something consistent.
FWIW the bullet is a 6mm Berger 95gr VLD shot through a 1:8 twist barrel at 3280 fps. Temperature was 80F at 3900 ft elevation
Where are you shooting? I recall the pictures on your website and it appeared that you were shooting from one ridge to the base of another ridge across a long cleared area. If you shoot from ridge top/side to adjacent ridge base across valley you could be in for some updraft and downdraft depending on the time of day and other factors. These up and down drafts could cause you considerable elevation deviations at those distances.