runout = shoot all day long....pretty soon you "run out" of bullets 
seriously the term runout refers to how far out of perfect alignement the bullet is with the centerline of the case (how crooked the bullet is in the neck of the case)
If you roll a finished round of ammo on a mirror you can see the runout quite easily. They also make different devices to measure this and there is a company that makes a deal to straightened out crooked rounds. I figure its best to just learn to load them straight to begin with and be done with it.
