In addition to what Unclenick said about groups opening up as range increases, there's another cause the not too many people realize. Ballistic coefficients across a box of bullets are not all exactly the same.
With small dimentional differences in jacket thickness and core location, all bullets are unbalanced to some tiny amount. The more unbalanced they are, the more they nutate their points around the trajectory path; they tend to take microscopic flight paths. That means the more they nutate, the more drag or lower ballistic coefficient (BC) they have. A 2 to 3 percent BC spread in hunting bullets is normal. A 1 to 2 percent spread in match bullets is normal.
Go run your favorite ballistics software for a given bullet at a given velocity and change it's BC by 2%. Compare the drop for each 100 yards all the way out to 1000 yards. That'll show you how much the groups open up for the velocity you chose. Then add the velocity spread your bullets leave at and the results may surprise you.
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Last edited by Bart B.; March 7, 2013 at 10:03 PM.