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How to Calculate the Ballistic CoEfficient of your pellets.

As a follow on from my Self Reliant Airgunning - Pellet Production post , I thought I'd show a quick method on how I've been calculating BC without getting too technical.

The theory is pretty straightforward. I don't have two chrono's, just a combro, so I am relying on Chairgun and its 'BC based on POI difference' calculator.

Ten shots in total were fired. 5 at the zero of 12 yards, 5 at 30 yards. This gives us two distances. The fall of the shot will tell us how the pellet is performing in the barrel, then the air as it reaches its target. Of course, the chrono is telling us how fast it is leaving the muzzle.

Now, this is important. The database BC was very different from my findings. This is due to manufacturers taking an average performance figure, in the same way they take an average pellet weight and print it on the box. It's convenient and accurate enough for most peoples needs. Evidently, not mine!

When I was weighing the pellets I made, then a manufactured tin, I saw how we take so many figures for granted with out bothering to check and measure for ourselves. So no wonder we all have those 'mystery' shots that take us aback. Especially when changing pellet brand.

After the shots were fired, the average fps was taken (703.9 fps with a 10fps variation possibly due to a strip down just prior to the test) then the groups analysed and measured.



Accuracy on the bull would come later, for now, a clear grouping was sufficient. (I couldn't help but fiddle with the windage, though not elevation)

I settled for a rise of 0.896" over 18 yards. (click images to enlarge)



My old zero was 12.1 - 27.5 yards before the strip down and the RWS Supermags were calculated to be 0.0062 @ NTP already a big difference in performance (or calculations!)




Here we have the new zero and a graph that should accurately reflect the performance of my rifle and the pellet of choice. Chairgun will then update the pellet database for you with your BC calculation.

Yet another variable in airgunning to consider, measure and adapt to. Who said it was all 'point and shoot'?!

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