when working up new loads, I usually just slightly over size a brass, making sure the brass length is at a standard and loosely seat a bullet, with no powder or primer, then I put this in the gun and close the action, the bullet hits the lands and is pushed backinto the brass, once removed from the gun I measure it and that gives me overall chamber length, and I work up a few loads with the only difference being the distance the bullet ogive has from the lands on the rifling. most of my loads in various gun usually wind up being about 30 thousanths off the lands, no overpressue issues and better accuracy. but every gun is different and so is every reloader
most of my work goes into brass prep and powder charge. I weigh each piece of brass and each bullet, looking for consistency, which I believe is the key to creating good groups