Re: Help with JBM
Doc, if the only BC you have is for a G1, then don't change to G7. While Ballistic (which, BTW, offers the JBM engine, but is not his product) will convert BCs between different drag models, e.g. from G1 to G7, that doesn't mean the converted BC is correct. For instance, a certain projectile I use has a G1 BC of .445 and a G7 BC of .228. However, Ballistic, when converting from from G1 to G7 reports it as .224 and when converting from G7 to G1 reports it as .454. That would make a difference.
So go with the G1 BC. Then, as suggested, check your other inputs. It is important to know muzzle velocity, temperature and either your station pressure (uncorrected, raw pressure data) or the corrected pressure, which must be combined with the altitude. Ballistic is somewhat confusing in its application of these variables because it has both a "Zero Atmosphere" and a "Current Atmosphere." Zero Atmosphere will only make a difference if you zero at 300 yards or more. But unfortunately, you need to use it. So fill in the details as best you can, selecting "Pressure is Absolute" ON if you have station pressure. You won't need altitude then. Otherwise, set it OFF and enter both the corrected pressure and the altitude.
Make the settings in Current Atmosphere the same. Once you're zeroed, you can alter settings in Current Atmosphere, e.g. temperature, pressure, etc. and leave the Zero Atmosphere settings alone.
Once you've established that you're entering the most accurate parameters possible and using the settings in your ballistics app correctly, then tweaking BC or muzzle velocity slightly to match your actual results is a decent way to go about it. Just try not to change more than one variable at a time or by much. Another good thing about having a 100 yard/meter zero is that it's short enough not to be effected by varying conditions. In other words, you can go back to the range another time and tweak things a bit @ 100, then go over to the longest range available, e.g. I will hit the 300 or 600, to verify the results.
