RO is Run Out, and is one of the achilles heals of reloading. No Run Out is good run out, even a couple of thousandths is too much to the serious long range shooter.
Do you know exactly what Run Out is?
I saw your reply about how to seat the bullet and in one instance you wrote you would implement the change, then in another you said you probably won't. I am guessing that the latter "won't" comment was on turning the necks.
You can get a great concentricity gauge from Sinclair International. It is my opinion that turning the necks on GREAT brass is a waste of time, but doing it on good to mediocre brass is worth it's wt. in gold in terms of consistent groups. But many do not do this step and it is quite frankly, personal choice. I do not turn Lapua brass, but I do the WW and Fed brass, and in most cases it has made a substantial difference. I like that.
You could surmise that turning neck brass is ~ equivocal to trimming the meplats. It just gives you that 2-5% improvement.
I'd guess that if you seat a bullet with one stroke in a standard seater die then you have lots of RO. And by lots, I mean upwards of 10-15+ thousandths measurable on a concentricity gauge. I've seen this with many of my OLD handloads and factory ammo. Which still puzzles me to this day how some stuff shoots as good as it does under 250 yards.
Anyway, good luck to you and if you take some extra time with brass prep and concentricity implementation, you'll enjoy your hobby much more.
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Another question, just how much will this effect grouping?
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Well, how much this affects grouping is a question that has many parts to the answer. It depends on the exact rifle you are shooting, how concentric and squared it is, if it is bedded properly, and if the action mated to the bbl. is fundamentally sound. Ultimately, with a well built rifle that had the chamber cut with a great reamer and the tolerances are close to zero, the loading steps I describe could mean the difference between a 1/2" rifle at 100 and a one holer at 100.
As of yesterday, I have now established 4 of my rifles to be one holers, but only 2 are consistent one holers---with 3 shot groups. This has taken me hours upon hours to achieve. But, I have hundreds of the bullets, and up to 8 pounds of the powders in each instance, so all is good, and hopefully repeatable, time and time again. Let me tell you, it is a **** good feeling taking a hunting rig to the range and the bench rest guy 2 tables down is ****ed b/c your "deer rifle" is more accurate than his $5000 bench rest 6PPC. And that is with Sciroccos and Btips and TSX bullets. A Juenke bullet gauge will tell you quite a bit about your bullets too. Surprisingly, the Scirocco is actually the most concentric factory hunting bullet (lead core) that I've seen.
Tell me how you resize your brass and how you measure the fired brass after it is resized, how you set up your sizer die, etc.