Re: Mixed Cases
DocEd, a 20 fps change in muzzle velocity won't change vertical group impact at 300 yards very much. For example, 6mm 70-gr. HP boattail bullets leaving at 3240 average fps will center about 1/4th inch lower than if they averaged 3260 fps. The two groups would probably overlap quite a bit, too. It would take a rifle-ammo-shooter system capable of producing no worse than 1/8th MOA (3/8ths inch) groups at 300 yards to quickly and easily see the difference.
Consult some ballistics tables to see what a 20 fps change in muzzle velocity will do to vertical group centering for different bullets leaving at different speeds.