To make long-range hits, you must true your muzzle velocity and use the correct B.C. We go over that in this episode of The Overwatch!

I use the B.C. from Applied Ballistics by Brian Litz. The B.C. is measured at Mach 1.2 or just before Transonic. Once you have updated your B.C., you will need to shoot out to your bullet's maximum effective range (Mach 1.2) or as close to it as you can get. In your ballistic calculator, you will update your drop chart to how many Mils or MOA you need. This will true your muzzle velocity. Then confirm from 100 yards out to distance.