Bullet Runout explanation

Through testing, we have found that Collet sizing a neck to .001 inch under the intended bullet diameter gives the optimum amount of bullet pull with the least amount of runout. 

One can order an undersized mandrel for a tighter fit on the bullet , but we have found that runout increases as bullet pull increases. There is a fixed amount of spring-back in the case mouth after sizing occurs. If you were to size a case to .005 under bullet diameter, then seat a bullet, then immediately pull that bullet, you would probably find that the case mouth would measure .001 under bullet diameter. 

The explanation is that the bullet essentially resized the case mouth as it was seated. Because cases are never perfectly uniform in neck wall thickness or hardness, when a bullet is seated in an undersized case mouth, it will push out more on the thin/soft side, resulting in bullet runout.