On MagnetoSpeeds, BCs, and JBM

[DrThunder88]

So, I'm going to a 1000 yard match next month and am working out my drop chart. The one Dan made for me at Bang Steel using JBM Ballistics was consistent with my load out to 1000 yards, but it was set up for Summer in the mountains of Virginia and used a placeholder velocity to match the observed elevations at various ranges. I'm sure the flatlands of Lower Michigan in Fall will not be too far off, but it raised the question of what my actual muzzle velocity is.

To that end I affixed my MagnetoSpeed Sporter to the end of my rifle and got some data. The rounds I used were from the same batch I took to Bang Steel. They consist of a 115gr Berger VLD hunting bullet over 46.0gr of H1000 in a Hornady case with a CCI BR-2 primer (1% over max, do not attempt). The temperature was about the same (mid 80s), though the altitude was only 1000ft ASL compared to Bang Steel's 2500ft. Speeds averaged consistently around 2890fps, far lower than the placeholder 2960fps.

I was dinking around with JBM using this new muzzle velocity and found the resulting trajectories did not match my observations. If I nudged the BC of the bullets up a bit (0.279 to 0.32) I would get much closer to the mark. Now I'm cool with using approximated inputs to generate outputs that actually work, but I'm left wondering where the inaccuracy is.

The JBM ballistic calculator itself would be my first guess, though I ran the same inputs through another calculator and got the same results. The MagnetoSpeed is another possible source. I don't know how accurate the system is, though its reputation is pretty solid. Finally, is it possible Berger didn't get the BC right? I know Sierra lists BCs at different speeds, so maybe they market the bullet with a lower BC.