Runout = Saturday afternoon doing yardwork, someone takes the last beer....you have "runout" and someone needs to 'runout' to the store and get more.

Concentricity of your ammo. Ifyouroll your loaded rounds on a mirror and you can see any 'runout'. Some things that cause runout

neck wall thickness irregularities
bullet seater not set up correctly
case not pushed into the shellholder all the way when seating a bullet