Well, sometimes. How's that for equivocating.
Once did an experiment with an accurate custom 7x57 with a short-throated chamber, using Hornady Light Magnum factory ammo. I bought three boxes and sorted them for bullet runout, divided the best, middle and worst batches into the three boxes. As I recall, the best box had .003" runout or less, the medium box, .004-005", and the worst box over .005".
The best box shot grouped three shots into 1/2" or so, the medium box shot around an inch, and the worst box into 1-1/2". That's pretty typical of a custom rifle, but most factory rifles have looser chambers, and sorting below .005" or so doesn't do much good.
In a really accurate varmint rifle sorting to no more than .003" will help, and in a real benchrest rifle runout of more than .001" (and sometimes even less) will normally show up on the target.
I've been playing with a 6mm PPC benchrest rifle for a few months now, and my Redding Competition dies produce ammo with no more than .0005" runout. With its best load, in neck-sized fire-formed brass, it groups five rounds into around .18" at 100 yards. With new brass the runout runs up to .003" and the groups open up to .3" or so.
