Date: Oct 31 2006
Perhaps the most basic principle of advanced rifle handloading is that the bullets should be seated concentrically with the case. If a bullet is seated slightly "crooked," it tends to strike the rifling at an angle. This not only places the bullet at a slight tilt when it exits, but also can deform the bullet enough to affect flight. Neither helps accuracy.
This is why most serious accuracy handloaders own a tool to measure bullet runout: how much the seated bullet deviates from the body of the case. The standard procedure is to measure runout about .1 inch from the case mouth. In big game rifles, .005 inch of runout starts to affect accuracy, while in varmint or target rifles less than .003 inch is the ideal. The big factor in seating bullet straight is the concentricity of the neck of the case to the body. With straight necks, bullets tend to seat straightly. Without straight necks, even the finest seating die in the world doesn't help all that much.
I've written elsewhere about how to buy or modifying dies in order to size cases so that necks are concentric with the body of the case, but even with straight necks, once in a while a bullet will still seat a little crookedly. If the amount isn't too much the bullet can often be straightened after seating. Let's say you're loading a batch of .30-06 loads for hunting, and while most of the bullets ended up no more than .005 inch out of round, a few are .007 or .008. If the bullet is pretty long, say a 180 grainer in a .30-06, you can often straighten it simply by pressing down on the side of the bullet while holding the case steady in the concentricity tool.