Dusty, You're right on as far as bullet seating goes. The length of the brass does not affect the cartridge OAL, only the bullet seating depth does. The length of the brass neck is what I was referring to. If there is an empty area in your chamber throat because the brass neck was too short, it allows the bullet to do the floppy chicken and not enter the bore in perfect alignment. This will affect accuracy. Are ya with me now?
firstshot, You might want to try annealing your cases. It makes the shoulder-to-mouth soft and has some good benefits. It makes the cases grow a bit faster, makes the cases last longer, and it releases the bullet before pressure peaks, thus better bullet-to-bore alignment and less "bullet strike damage". Accuracy is improved considerably with no change in velocity. If you look at your new RP cases (or Lapua), you will see a pink hue and some heat discoloring. This is from annealing. All factory ammo and new brass comes annealed. I find annealing every other loading really improves accuracy. Your cases will grow and fit way better.
When it comes to bullet seating depth, most reloaders find a nice accurate powder charge then play with the seating depth. This is backwards and will make you chase your tail. As the bullet depth is changed, so is the pressure curve, thus velocity and accuracy. If you set your bullet to .010 off the lands, then play with your powder charge, you will find the most accurate load possible. Keeping the bullet close to the lands minimizes bullet strike damage and keeps the bullet nice and balanced. Each different bullet type will seat at a different depth. This is because the ogive can vary considerably with weight, length, and nose design. The bullet seating depth tool will measure this for you no matter what bullet you use.
Last but not least is the rifling twist rate. If you want to optimize accuracy, you must match your velocity to the twist rate in order to keep the bullet stable down range. Most any bullet will maintain stability for at least 100 yds. After that, if the bullet isn't spinning fast enough, it will start to wobble and loose accuracy. If you see oval holes, or worse yet, keyholes in your target at 200 or 300 yds, it's a sure sign your bullets aren't spinning fast enough.
The bullet spin speed is determined by the rifling twist rate and velocity. You can't change your rifling short of changing the barrel so you must adjust your velocity. If the velocity is too fast, the bullet will skid across the lands and grooves and act like a file. This will be evident when you see excessive copper fouling in the barrel and spread groups. If the velocity is too slow, the bullet will wobble. The formula for velocity range in a 30-'06 is: 250 times twist rate, plus or minus 10%. Example: if your twist rate was 1:12, then multiply 250 times 12, or 3000 fps. Your velocity should fall between (.9 times 3000 = 2700; 1.1 times 3000 = 3300) 2700-3300 fps with 3000 fps as optimum. Next you have to find a bullet weight and powder charge that you can safely drive to velocities within that range.
Sorry for the long post but if you want to squeak some better accuracy out of old Betsy, give these techniques a try.
