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The 'ballistic coefficient' or bc is a measure for the drag experienced by a bullet moving through the atmosphere, which is widely used by manufacturers of reloading components, mainly in the US. Although, from a modern point of view, bcs are a remainder of the pioneer times of exterior ballistics, ballistic coefficients have been determined experimentally for so many handgun bullets, that no treatise on exterior ballistics would be allowed to neglect it..
The bc of a test bullet bctest moving at velocity v is a real number and defined as
the deceleration due to drag of a "standard" bullet
The standard bullet is said to
have a mass of 1 lb (0.4536 kg) and a diameter of 1 in (25.4 mm). The drag
coefficients of the standard bullet can be derived from the G1-function given
and will be named cDoG1(Ma) .
cDotest(B,Ma) = iDtest(B) * cDoG1(Ma)
one finds for the bc (assuming "standard" atmosphere conditions)
bctest=1 / iDtest(B) * mtest / d2test
This formula also shows that the bc and the form factor iD of a "test" bullet are two aspects of the same principal simplification: the substitution of the (unknown) particular drag function of a bullet by the (given) "standard" drag function of the standard bullet (see also here).
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