Explain?

When you fire that big boy off, you want that bullet to at least enter the barrel straight. A little runout probably won't hurt when hunting unless you are reaching out to touch someone.

For targets, it might be the difference between that one holer and that flyer.

Well, anyway, that's how it see it.

It is a measure of the offset between case and bullet longitudinal axis. Equipment that measures RO typically allows the case to roll while resting a micrometer on the bullet which gives you a measure of offset espressed in thousandths of an inch. This measurement is entirely relative to the position on the bullet you are taking the measurment from and as such is not an absolute measurement. True evaluation of this would be expressed in degrees or fractions thereof, not inches.

The more misaligned the bullet is, the more likely you are to induce dispersion in your groups, or from point of aim. Benchrest quality ammo has no RO, Varmint hunters often accept up to .002-.003", Big game ammo maybe up to .005. My personal standard for all hunting ammo is .003" or less, measured approx. 1/8" from the case neck.

It is likely that you will find some degree of RO betwixt case body and case neck as well. There are ways and means for dealing with all of these issues. You never really learn to load quality ammo until you start futzing around with this. It is also an excellent way to drive yourself insane. My first and only tip on this post: Virtually all factory ammo has RO to some degree, so does most handloaded ammo. If it ain't broke, don't fix it.

Lots of earlier discussions. Do a Search under "runout" and "concentricity". It's been chewed on pretty thoroughly in the recent past.

I think if the bullet enters the barrel crooked, the center of angular momentum [the line about which the bullet wants to spin] is not in the center of the bore. When the bullet leaves the muzzle, the bullet is free to move in the direction of it's center. The calculation is that displacement is about 1" at 100 yards for .004", 1/2" for .002" runout etc. The government, the NRA, and A.A. Abbatiello found the calculation to be predictive up to .004", and any worse than that has no additional effect. This is presumed to be because the throat will not allow any more than .004" on bullet misalignment.

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