Most of what RePete wrote I agree with. We differ a bit on the trimming, but only by a few thou.
Other than using bullets that the rifle likes (mine tend to like Lapua, Berger and Hornady), seated at the depth that the rifle likes (some like them jammed into the lands, some like a 0.020" running start and others are in between), the key to accurate ammo seems to be ...
Strive for a total runout of zero; but accept nothing over 0.002". Buy a concentricity indicator from Sinclair or Forster. It's a very useful tool.
Doing nothing other than sorting your ammo with this tool will make your groups more predictable by eliminating some of thos 'unexplained' flyers.
"Those who have long enjoyed privileges as we enjoy, forget, in time, that men have died to win them."
Franklin D. Roosvelt