A brief definition: runout simply means that the bullet is seating (or appears to be seated) in a crooked, or canted fashion with regard to the rest of the case. When the round is chambered, the bullet does not point straight into the chamber in a concentric fashion, but rather off center. When the round is fired, the bullet impacts the throat area off center, but it of course is corrected (to some extent) by the throat, and it continues on through the barrel. Runout may cause the bullet to engage the lands off axis, and it'll spin down the bore with its tip drawing cute little circles in a corkscrew pattern...
The question as to how much if at all runout affects final accuracy is truly hard to answer conclusively. Weeding out all of the other variables is difficult if not impossible, and it would likely take thousands of rounds fired by various shooters shooting various bullets through various rifles to ever come to even a tentative conclusion as to how much
"X" amount of runout affects this rifle
shooting this particular bullet
set at this particular distance from the lands
at some pre-specified range. Quite a conundrum if you ask me...
Runout is never a good thing--that goes without saying. But it's often not such a terrible thing either. If one thing can be said with absolute certainly regarding runout it's this: The actual, real world effect of runout will always
be only a small fraction of the effect that the folks profiting from gizmos and gadgets designed to reduce runout will ask you to believe. [img]/ubbthreads/images/graemlins/laugh.gif[/img]
But on to the point. There are different types of runout.
Treating all types of runout as the same will likely have you chasing your tail with those hundred dollar dies and press manipulations and ball micrometers and Bersin tools, et al, ad nauseum, until you're so old you don't care anymore~!
One of the worst and most common types of runout results when the bullet is seated askew. Not only does this start the bullet into the bore crooked, but it would also seem to have an effect on neck tension as well. Rounds of this type might be the culprit when groups go awry, and the only thing that has changed is the average runout.
Another type of runout is what I would call "case wall induced" runout. You often see this with new, virgin brass. The case wall is not concentric, even after being full length sized. The sizing die has brought the case as close as it can to uniform, but there are still imperfections that will only be remedied by fire forming. (You can check for case wall runout by putting the needle on the case wall, and rolling the case. This can be a real eye opener on some batches of new cases! ) But this type of runout may not really be runout at all. When you place the finished cartridge on the concentricity gauge, the out of round case wall causes the gauge to read as if the bullet is seated askew. However, it may well not be. Once this round is chambered the case neck and bullet may align near perfectly with the throat.
Case neck thickness induced runout is a biggie. If the case neck is thicker on one side than on the other, it will cause the neck to be out of line with the case body when the case is sized. Seating a bullet into such an abomination will result in a cartridge with excess runout, and fire forming will not solve this problem. Neck turning, or trashing the cases (my choice) are the only solutions. You will usually be alright with about .003 to .005 inches of case neck thickness disparity (for the average, 1/2 MOA rifle), but no more on calibers under thirty or so. By the way, .005" of case neck thickness variation will not necessarily turn into .005" of bullet runout.
Yet another type of runout that your concentricity gauge can go bonkers over is "case head out of square" induced runout. In this scenario, the case head is out of square with the case body. This is sometimes seen on virgin cases, and also on fired cases coming from rifles with out of square bolt faces, or uneven bolt lug engagement.
Since the case head is held against a stop post as the case is being rolled, if the case head is out of square the cartridge will oscillate front to rear. If the runout gauge's needle ball is sitting on the ogive of the bullet, considerable variations will be seen on the dial. While this type of runout may be only realted to the out of square case head, it is likely that the case is sized out of spec as well, due to the effect that the askew case head would have on alignment during sizing and bullet seating.
Lastly (though there are probably other forms I've forgotten to mention) is bullet runout. This means just that--the bullet itself is poorly made and not concentric. Rounds with this type of runout probably fare worse than all others combined.
My point in all this is that before you can say unequivocally that runout does or does not affect your rifle's accuracy, you have to identify the type of runout you mean. Case wall induced runout may have little if any effect on groups. Someone shooting a box of such cartridges might conclude that runout is "no big deal." On the other hand, severe case neck thickness inconsistencies , or crooked seating of the bullet will likely affect the groups to some noticeable degree.
No matter the pedigree of your loading dies, if your cases have poor necks, you can't fix them. If your bolt isn't lapped and only one lug is bearing (Remington700itis is my word for it [img]/ubbthreads/images/graemlins/laugh.gif[/img] ) you may not be able to get good numbers on the concentricity gauge--even after resizing.
The bottom line is this: If you are concerned about the amount of runout you're getting, you should first do some testing to see how much (if any) the amount of runout you're dealing with is affecting accuracy. The proper way to conduct this test is with a Blind Bag Test...