To minimize extreme spread, all things must be consistent.
I would start with weighing a 'set' of cases from a single lot number. If they all weigh the same, they should have the same volume.
Then weigh your bullets. For this part of load development, I don't have a 'range' of acceptable weights. All bullets chosen will weigh exactly the same.
Then (of course) your powder charges must be individually weighed...and care taken to be sure every granule of powder ends up in the case, not stuck in the drop tube or something.
Consistent neck tension is required, or no neck tension at all. I choose 'none'.
After all rounds have been assembled, they should all weigh the same if the individual components did.
Finally, barrel condition must be the same for each shot. How you choose to do that is up to you, but wiping between shots seems like the most certain method.
I have seen ES and SD numbers less than 10 using only the measures above. There are other things which can be added...uniforming primer pockets, to name one.
Retired...twice. Now, just raisin' cows and livin' on borrowed time.