I'm just glad to see you've done your homework in developing the perfect load for you & your slug gun. Sadly, most people don't even go to that kind of trouble with centerfire rifle/load. You're also correct that factory dimensions & tolerances is the reason all guns are slightly different. I'm not used to microns form on measurements but most factory barrels will vary up to .002 in chamber/throat/barrel tolerances, with some brands/quality levels naturally being closer than others. I've always dreamed of being able to make my own barrels but will have to be content to order from the top quality barrel makers and machine to final dimensions. I'm sure if I ever did get into barrelmaking, I would lose the few hairs I have remaining on my forehead.