I have been looking at a lot threads regarding neck sizing and frequently come across a few terms that I have question about

bullet runout
case runout
concentricity

Is there a picture that helps describe what these are and how they are measured?

If you place a loaded cartridge on a table and look at it directly from eye level, the tip should make perfect circles when rolled with your fingers. In reality most don't. The degre that they're out of line is what's termed "runout". Concentric is the opposite. When the tip of that bullet doesn't vary when rolled, the cartridge is said to be concentric. It should not be more than about .002 for acurate reloads. It stands to reason that if a cartridge case has runout, the completed round will have at the minimum that degree of runout, probably more. This is why some reloaders use Bushing dies or dies that allow no tolerances like Wilson. The bullet itself will have litle runout usually. The major culprit is the expander ball in your die. It is pulled back through the case to open it enough for bullet seating. The strain or stress put on the case neck is enough to throw it out of kilter. Some reloading manuals will clarify this and explain it better. Best wishes.

Cal - Montreal

The term "concentric" means having the same center.
If a cartridge is spun in a circle with the case as the bearing surface, then a test indicator or dial indicator on the bullet will show some movement. That movement from greatest to least over the 360 degrees of rotation is the run out at the bullet.

The NRA and A.A. Abbatiello wrote an article that says mathematically .004" of runout [less than the eye can see] will cause 1.1 minutes of error in the direction the bullet takes when exiting the muzzle. If I understand that, then +/- 1.1 minutes would be about a 2.2" group at 100 yards in a rifle that could shoot perfectly.

This is what I have.

from http://www.sinclairintl.com/
In the instructions, it says the expander ball is probably the problem. That was most of the problem. The smaller problem is cheap seater dies. Forster benchrest or ultra seater, Redding competition, and Wilson seater dies all hold the case in place laterally while the bullet is held in place while the two are brought together axially.

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