We have all seen or heard of some incredible shots made with standard factory rifles and fairly generic optics. However, while these shots are possible, to repeat them with any degree of consistency is another matter entirely.
Arguably the most important factor expediting long-range target engagement is bullet design (read ‘shape’ or ‘config- uration’). With the aid of modern electronics and gizmos it is no longer difficult to determine the precise or near-precise distance to a target. What remains a challenge is the influence of the wind, as this can only be estimated for the full flight period of the projectile. A bullet of highly aerodynamic configuration can limit this unknown influence to a certain extent, being less easily deflected from its course by wind. Bullet aerodynamics are determined by modeling or calculation from measured velocities and reflected as a ballistic co-efficient (BC).
There are numerous measurement scales designed to be accurate for a specifically shaped projectile; these are identified as G1, G5, G7 etc. These Form Factors were developed to meet the ballistic requirements for artillery fire and the estimation of impact points for long-range shelling. This techno- logy was applied to small arms and has been developed into an almost exact science with highly predictable results.
As can be seen on the left, the G7 standard projectile shape is a much closer approximation of the shape of modern rifle projectiles, whereas the G1 shape is similar to a handgun projectile, flat-based rifle bullet or older styled cannon shell. The two standards indicate a BC factor to determine the velocity loss over distance, and in both instances a higher value expressed as a factor of 1, would be the most desirable characteristic. The BC of a bullet takes into account the form factor and the mass of the bullet, allowing comparison between different projectiles of the same calibre and same mass.
The limitation of the G1 co-efficient is that it cannot accurately predict the velocity change (loss) resulting from a long flight path, and to overcome this, some software applications allow for stepped changes in the BC, which then adapt the calculated bullet path to more accurately follow the actual bullet path. The G7 co-efficient, on the other hand, is a very accurate reflection of the bullet shape and the calculated path does not differ measurably from the actual, hence is easier to work with.
Clearly, therefore, when using a Ballistic Calculation or App, it is important to ensure that you use the correct co-efficient as required by the program. Using a G1 co-efficient in a G7 calculation will indicate understated adjustments for elevation and windage and vice versa.
Making a more practical application of the above, we ask the question, “How does the BC factor assist us to make a decision on calibre and platform (read ‘rifle’) selection?”
Read the full article in the August 2018 issue of Magnum.