BC=Ballistic coefficient

Cd=Zero-yaw drag coefficient

M=projectile mass

D=Projectile diameter (caliber)

P=air pressure

T=air temperature

H=air humidity

shape=Geometric shape (form) of the projectile

u=projectile velocity

us=sound velocity at the projectile's atmosphere

u/us=projectile velocity/sound velocity ratio

SD=Projectile sectional density

i=form factor

**Are the following points true or false ? (with the approximations used in BfX) **- us(P, d

_{air})

- d

_{air}(P,T,H) =>

**us(P,T,H)**- BC(M, D, shape, u, P, T, H) or

**BC(M, D, shape, u/us)**- Cd(shape, u, P, T, H) or

**Cd(shape, u/us)**-

**SD=M/D**^{2}-

**BC(M, D, shape, u/us)=SD/i(shape, u/us)**-

**i(shape, u/us)=Cd**_{projectile}(shape, u/us)/Cd_{standard projectile}(u/us)- Standard Gx projectiles always have diameter 1" and mass 1 lb

- A projectile with the exact same shape with the standard projectile (of the Gx drag function), but with different dimensions and weight, has a BC=SD at the same drag function, at all velocities

- Two projectiles with the same shape but with different dimensions and weights have the same Cd in the same velocity and at the same atmosphere.

- Lets say that we have a projectile with unknown BC ... the right way to build the BfX model for it is:

1. Compare the shape of the projectile with the Gx shapes and select the one more close to it.

2. Measure the BC near the muzzle (higher velocity with e.g. velocity differences at 2 points) according to Gx drag model selected in step 1.

3. Build the BfX model with the BC measured in step 2 and the drag function selected in step 1.

- Last question: why the BfX custom drag functions need as an input the length of the projectile ?