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.
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 three 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.