Kraig's right about spin drift. It's not much as he says. For example, a .30-06 with a 172-gr. FMJBT bullet leaving at 2700 fps has about 7 inches at 1000 yards. If you have everything perfect and this is what you get, then set your sights between 1/4 and 1/2 MOA left for a zero and the error for drift will be no more than 3.5 inches at 1000, half that at 600, 1/4th at 300 and so on.
If you can judge cross wind values from zero to 1000 yards across your bullets trajectory path accurate enough to make exact wind corrections with no more than a 1/8 MOA error, then you need to be concerned with spin drift. To date, I don't know of anybody that can do that.
One other thing; you and your shooting stuff has to do no worse at 1000 yards than 3 inches maximum spread all the time or you will never see the effects of spin drift.
Last edited by Bart B.; November 25, 2012 at 08:47 AM.