# BC Decay Rate

#### cronhelm

##### Well-Known Member
Warren,

I was looking at your .408 Cheytac test reports today. Neat toy, can I borrow one to test? I'll write a nice article in my magazine for you.

In the report I found a reference to "BC Decay Rate". This is something new to me.

Is the decay rate pretty much the same for most bullets or is it highly dependant on shape, calibre etc?

Is there are way to calculate expected decay rate or is it possible to estimate the decay rate based on calibre, weight and shape?

This is what I love about the sport of shooting. There is always something new and fascinating to learn

Peter Cronhelm

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.

Warren,

We should start calling you "Answer Man".

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

Peter Cronhelm

Peter

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.

Just a thought.

Dave,

You are right, the pictures are of the 1900yd shot when the bullet hit approx 23 MOA below the calculated value. This shot was into a 20mph headwind

The 1500yd shot was over almost flat ground and in much hotter temperatures. The bullet hit approx 8 MOA high. This shot was in a 5-10 mph crosswind.

Obviously I need to get out and do this some more so I know what to expect out of my equipment.

Do you think the wind would make THAT much difference over the extra 400 yds?

Peter

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