Ballistic coefficient calculation
Commonly used nowadays is the ballistic coefficient (BC or G) according to Karpov (i.e. Dr. Boris Karpov, US Army Research Laboratory, 1944), which represents not only the characteristics of the shape and the weight of the bullet, but also takes into account the actual air resistance at a specific velocity.
The number following the letter indicates the distance for which the coefficient is applicable, in hundreds of metres or yards.
To calculate the ballistic coefficient requires two velocities. The initial velocity (V0), and then, at a certain point on the trajectory at the distance x from the muzzle, the Vx velocity. To measure V0 directly is difficult; therefore V5 and V100 were measured, from which V0 was subsequently extrapolated. The ballistic coefficient for one hundred metres is calculated using the following formula, in which x = 100 m. Similarly this is also applicable, of course, to other distances.
The ballistic coefficient was not adjusted in accordance with the altitude and should be considered as being universal, since Sellier & Bellot's ballistic testing laboratory is 400 m asl., which corresponds to the average altitude of the Czech Republic. A distance of one hundred metres was chosen because, at this distance, S&B also checks the accuracy of the ammunition and because this distance has long been considered as being a “hunting” distance. The actual tests were carried out in such a manner that, using the same series of ammunition, first V5 and subsequently V100 were shot from the same barrel. Since the shooting was not performed simultaneously, firing was repeated with other series and the uniformity of the results was monitored. The ammunition was always tempered to +21 ºC.
BCs are calculated to two decimal places, which is in practice is completely satisfactory. Even after correcting for the atmospheric conditions and the altitude, the weight tolerance of the bullet and its initial velocity (V0) still come into play, and of course so does the length and the wear of the specific barrel.
The entire issue is actually much broader, because, for example, on the Internet it is possible to find articles that describe differences of up to 25% from the values reported by the manufacturer and discovered in the case of overshoot.
The values specified by S&B correspond very well with reality, because the velocity at a distance of 100 metres was measured and also because it is not a result that is based on a single firing. Also the documentation provided by the Sierra Company can be accepted as being reliable. The results, however, for the reasons described in the preceding paragraph, cannot be taken as dogma.