If you look at a chrono, some barrels shoot good low std dev. velocities with a clean barrel and dirty. Some barrels, mostly factory barrels, need 2 or so rounds for the velocity to start settling down. The problem is the oil we put down the barrels. If I go somewhere where long shots are possible, I like to have a dirty barrel with 2 shots thru it. As for sighting in, I say go to the range and shoot a box thru it. Find the zero. And don' t worry about it.