RE: .204 Ruger for small varmints
BC - one of the central principles of ballistics is that a bullet of a certain ballistic coefficient, launched at a certain velocity, will maintain the same trajectory and time of flight out to any distant target - regardless of bullet weight. Bullet weight does, however, affect the amount of wind drift.
For instance; the 50 gr. Sierra .224 spitzer and the 150 gr. Sierra .270 RN both have a ballistic coefficient of about .250. The wind drift at 500 yards is 25% more with the lighter bullet. It only gets worse as the distance increases.