I use the method that Red Dawg describes. Once I have that measurement, I subtract .015 to .030 from the case overall length just determined for that gun. Once you subtract that, load your re-loads to that new shortened length. You want the bullet just off of the lands (Rifling), not touching them. I am sure some reloaders do not subtract as much as I do, .015 to .030 is what Nosler recommends.
The new COL that you just found for your gun might not fit in your magazine, COL is too long. So load to your new COL and run a magazine full of ammo through (not firing it) to make sure it operates smooothly in your magazine. My 30-06 has a longer COL than the magazine will allow, but my 338WM it works just fine. If it is too long then keep slowly shortening the COL until it will run smoothly through the magazine.
If you use this method, you will get better tighter groups than using NAAMI published COL (at least that has been my experience). NAAMI is what all of the reloading manuals (at least the ones I know of) use to publish their measurements of cases.
