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What is BfX?BfX is a tool for the sporting shooter who, from time to time, has to do some calculations. BfX is accurate as long as the elevation is between -15 and + 15 degrees. This is typically the case for a target shooter. This restriction allows one to make the so called flat fire approximation which results in mathematical formulas for bullet trajectories and stateless algorithms.
A stateless algorithms does not keep information about the calculations done. Once the algoritm is finished, its memory is reset. These algorithms are very suitable for the Excel context.
BfX is checked against models that do not apply this approximation, e.g. a 4th order Runge Kutta solver. The outcome of such comparisons is that if you stay below 15 degrees BfX works fine and is accurate enough. Enough means the following. If you do a calculation with BfX or a numerical solver for a trajectory to hit a target at 1000m that is right in front of you with a 740 m/s, .308 185grain Sierra Match King bullet the differences are less then the uncertainties due to the input of the calculations. How accurate does a chronograph measure the bullet speed? How well is the ballistic coefficient known? How well is the drag function measured? Near the velocity of sound?
Supporting the Runga Kutta or even the so called 6-degrees-of-freedom methods in Excel is certainly possible, yet unnecessary for competition shooting. The physics of BfX was explained in the book "Modern Practical Ballistics" of the former Arthur J. Pejsa and here. There where quite a few typo’s and unnecessary approximations therein, which found not their way into BfX. Pejsa supplied also a drag function that should be used with the G1 ballistic coefficient and this is the default drag function used in BfX. BfX also supports other drag functions, amongst which the G7 drag function. The G7 drag functions can be used in combination with the measured G7 ballistic coefficients that you can take, for example, from the book of Bryan Litz "Applied ballistics for longe range shooting". Furthermore, BfX contains a drag function termed "A" that was comparable with the one deployed by Chairgun Pro, a popular ballistics calculator of the airgun community. Chairgun is not available anymore.
BfX also supplies an interface to Visual Basic for Applications that is part of MS Word, Excel, Access etc. With this you can do your own programming.
Where can I get help?Most explanation about the use of BfX is done in the workbooks you can download from this site. You might also consult other users via the forum. BfX itself provides help in Excel, use