Is this really so "unknown"?
The techniques for loading accurate benchrest rounds have been known and written about for a long time: Brass of consistent weight and dimensions, especially neck diameter. Bullets that are consistent in weight, and more importantly dimensions, especially neck thickness. Powders that perform consistently in different temperatures (a full case helps, even with other powders) and sometimes playing with primers to see which one results in the finest accuracy. (Yes, this still CAN make a diffference.) Then there's playing with seating depth, which often is more important than powder charge.
Plus, there;s the chronograph factor. Some provide more accurate results than others, but over the past year I ran a test with several chronographs ranging in price from around $100 to $500+. All except one resulted in the same average velocity, within less than 10 fps. This was while chronographing the same loads at the same time, with the chronographs set up together.
