when a bullet is not seated perfectly straight along the center of the case, there is run out.<P>i don't think you will achieve top accuracy if the ammunition you start with is crooked. <P>the rcbs casemaster can help you determine how much run out you have. also, rolling your loaded cartridges over a mirror will show if you have run out (look for the bullet tip to wobble).<P>when i seat a bullet, i lower the handle half way, or a little more, rotate the cartridge 180 degrees, and then finish seating. seems to produce pretty good results for me.