That explains it

When grabbing the link to the Berger VLD bullets to make the previous post something jumped out at me. The ballistic coefficient (BC) on my favorite bullet has been changed. It used to be listed as 0.640. They now list it as 0.631.

The first time I fired my rifle at 1000 yards I entered the temperature, air pressure, and wind speed/direction (I already had the scope height, muzzle velocity, BC, and inclination entered), into my little calculator. It reported back the sight angle for my scope, I tweaked my scope, and I happily aimed dead on and put my first three rounds into the bottom right of the X-Ring. I wasn’t surprised my wind estimation was a little off but why the bottom of the ring? Since then I’ve had the nagging suspicion that the algorithm used in the calculator and Modern Ballistics wasn’t quite right. Yes, it was close enough for all practical purposes. I couldn’t argue with a X-ring hit at 1000 yards on the first shot from the gun beyond 200 yards from a cold clean barrel. But as years went by it always seemed the gun and cartridge was shooting just a tad low from what I expected.

Running the numbers through Modern Ballistics tells me the lower BC gives the bullet another 2.5 inches of drop and an inch of windage to the right under those conditions. Not quite enough to fully explain my results but enough that it accounts for 50% or more of the error. That gets us into the 1/8th MOA range. This is well into the “noise” of shooter ability, bullet jacket uniformity, muzzle velocity variations, and to the point where you have to start worrying about the direction of crosswinds relative to the direction of the spin of your bullet and Coriolis effects–which requires you to know your latitude and the direction you are shooting.

So with the updated BC my little calculator and Modern Ballistics are, as they say, good enough for government work (back when I worked at PNNL I wrote a proposal and made a presentation to Special Forces about the calculator program for their snipers).

2 thoughts on “That explains it

  1. Wow. Is that how military snipers do it today? With ballistics calculators? Do you know if the PPCLI in Afghanistan used one to achieve that record shot?

  2. To the best of my knowledge they don’t currently use calculators. I’ll ask Eugene Econ at Boomershoot and see what he says. He teaches snipers at Fort Lewis.

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