JBM b.c. calculators

den

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Nov 25, 2007
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Oregon
Does anybody have any experience using the jbm b.c. calculators do they work ok? could I carefuly chrono @ 10 ft then @ 310 soon after in the same conditions with the same chrono and input these into the jbm and expect to get a fairly accurate b.c. reading? , thanks
 

bruce_ventura

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May 22, 2011
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Ventura CA
That feature is built into the Ballistic app for iPhones, which is base on the JBM engine. I've used it to fit a BC to bullet drop values and it works. It's only as accurate as your chrono or bullet drop measurements, so you need to confirm your results. Plus, it's only accurate over a limited range. I prefer to measure bullet drops instead of bullet velocity because I can do that easier at long range.
 

den

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Joined
Nov 25, 2007
Messages
407
Location
Oregon
Thanks for the response Bruce, I need to get some kind of handheld field device someday soon only have a dumb phone still , anyway went out and tried this today and well I must have done something wrong but I cant figure what ,I got 3240,3257,3204 @ 10 ft and 3140,3125 @ 290 ft chrono seperation or 100 yards from muzzle with a 175 gr rbt bullet in a 12 twist 300 rum jbm says g1 .801 ? when I shot these at 350 yesterday I was using a g1 of.550 bc and had to drop from 3 moa elev to 2 to get down on the target for ladder testing I think I read on here someone saying that their 12 twist 300rum did weird things with 178 Amaxes IDk I will try to do a little more shooting & measuring ect... hopefully Saturday
 
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