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 bc^test moving at velocity v is a real number and defined as

the deceleration due to drag of a "standard" bullet
devided by
the deceleration due to drag of the test 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 in literature and will be named c_Do^G1(Ma) .
Using

c_Do^test(B,Ma) = i_D^test(B) * c_Do^G1(Ma)

one finds for the bc (assuming "standard" atmosphere conditions)

bc^test=1 / i_D^test(B) * m_test / d²_test

This formula also shows that the bc and the form factor i_D 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).