This article was edited from software documentation to encourage a discussion about ballistic coefficients and explain why good B.C.'s are crucial to getting accurate results from ballistic software.
By the middle of this century rifle bullets had become more aerodynamic and there were better ways to measure air drag. After W.W.II the U.S. Army conducted experiments at their facility in Aberdeen, MD to remeasure the drag caused by air resistance on different bullets. They discovered the air drag on modern boat-tail bullets increased substantially more above the speed of sound than previously understood. They also learned further refinement to the original Ingalls model would better forecast the trajectory of flat-based small arms bullets. In 1965 Winchester-Western published air drag data based on the Aberdeen research for different types of bullets. Included was a modified Ingalls/Mayevski drag model designated "G1" and one for modern boat tail bullets designated "G5". The "G1" drag model has been accepted by ballisticians as the best "all-around" model and is most commonly used to generate published trajectory data. "G5" is primarily used for long range boat tail trajectories (1000 yards or more).
The firearms industry has developed myriad ways to compensate for this problem. Some bullet manufacturers publish trajectory tables that group bullets with similar aerodynamics under an "average" of calculated B.C.'s for "normal" velocities. These published B.C.'s are typically smaller at high velocities and larger at low velocities than the actual B.C. calculated from firing tests. Another way to estimate B.C.'s uses only the form factor of a bullet and a standard industry bullet classification. B.C.'s provided without trajectory data are often calculated using this method. It gives bullet manufacturers who do not know how the bullet will be used a way to compare aerodynamics but is at best an approximation and can be more than 10% off.
Some ballistic programs adjust published B.C.'s for velocities above the speed of sound or use several B.C.'s at different velocities. While these approaches mitigate some of the problem, in the final analysis, published B.C.'s are still not correct unless your gun shoots the bullet at exactly the same velocity used to calculate the coefficient.
There is another problem with traditional ballistics modeling. The change to air drag does not happen abruptly at predetermined velocity zones. Air drag change as a function of velocity is continuous with only small variations immediately above or below any point along a trajectory. Programs that translate Ingalls or 'G1' drag models directly to computer or use several B.C.'s change air drag values abruptly at pre-determined velocity zones and produce discontinuities. The calculated velocities in each zone do not duplicate a "real world" trajectory and is why many ballistic programs are not accurate when transversing the speed of sound.
A good ballistic program should be able to use two velocities and the distance between them to calculate an exact ballistic coefficient. This method of calculating a B.C. is much preferred and can be used to duplicate published velocity tables for a bullet when the coefficient is unknown or to more accurately model trajectories acheived from your own firearm. A lot has changed in shooting software. If your software is more than two years old, chances are it does not employ the latest modeling techniques or calculate B.C.'s. You can find both PC & Macintosh software that use the latest exterior ballistic modeling techniques at commercial sites on the Internet.