Re: Whats up with my BC?
Lots of variables here to screw your readings up, so you need to eliminate some variables. The issue is more than likely with the two different chronographs and possibly the distant measurement between the two chronographs. I utilize two chronographs for calculating the exact BC’s of my bullets. I use two CED Millenniums chronographs which when put back to back at 7 feet from the muzzle have an average 3 fps difference in velocities, which is very accurate. I also use a distance measuring wheel for measuring distances.
Let’s say for the sake of arguments your shooting chrony is working fine and your Competition Electronics ProChrono is working fine. Being two different brands of chronographs each company more than likely calculates the bullet velocities differently. More than likely each chronograph is giving different velocities readings which are screwing up your results. BC’s do change for a bunch of different reasons, but the change is usually slight and no more than a few % points. Your BC is not going to change by 100+ points. It can't it all comes down to math and physics.
Try using only one chronograph (I’d recommend the Competition Electronics ProChrono) and set it up 7 to 10 feet from the muzzle and shoot a string of 10 rounds to get a consistent average muzzle velocity. Measure accurately to a 100 yards (not with a laser range finder) and move the same chronograph out to the 100 yard line and shoot another 10 round string to get a consistent average 100 yard velocity. Now recalculate your BC, which should be more in line with the published BC.
Hope it helps
Distance is not an issue, but the wind will make it interesting!