I think there is more variation`s if your using a progressive press to load with anyway. Once the dies are locked in, they`re pretty much gonna stay put, but the issues will usually be the sub plate on the press itself. They ALL flex. You cant stop it either. Bullets arent perfect either. Bottoms of the cases after first firing arent perfect anymore either. I can understand wanting super accurate load for a benchrest rifle, but a 9mm pistol round, nah.