Use your bullets G1 B.C. to get your gun's wind number.
EX:
.308 - 175 SMK G1 B.C. - .505 = 5 MPH GUN
6.5C - 147 ELD-M G1 B.C. - .697 = 7 MPH GUN
In this episode, I explain the error with the bullet's "true B.C."
and how to correct that if it's needed.