The Ballistic Coefficient used mostly (abbreviated as C) is calculated as a pressure in pounds per square inch. Others may use a ballistic coefficient calculated in metric units as kg per square meter. We refer to the metric value as BCM.

In printed matter one might find a 3rd type. This is referred to a the Siacci ballistic coefficient as we use c as reference to this method. Siacci was a well known in ballistic mathematical models.

C and BCM is calculated as m/id^2 while Siacci c is (i d^2 1000)/m

Converting 1 pound per 1 square inch to 1 kg per square meter, results in a factor of 703.069

As example, given a C of 0.12: 703.069*0.12=84.37 as BCM and vice versa (84.37/703.069=0.12)

The factor from C to c (Siacci) is 1.422334331

As example, C=0.12 then 1.422334331/0,12=11.85 and 1.4222/11.85=0.12

It is important to know which is used, it is not always obvious.

To illustrate the importance: The BC of say the 122mm cannon might have a BC of 0.5.

Using that value shows a maximum range of about 5km, which is wrong.

But 1.4222 / 0.5= 2.844, which is the “real BC” as imperial value.