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In the world of ammo making, runout is the measure of co-axial misalignment of the cenerline of the bullet/cartridge to the cenerline of the bore in the barrel. It is measureable in the finished cartridge using a runout gauge. In layman's terms, it's wobble.

If you roll your ammo across a mirror and look closely at the tip of the bullet and its reflection you should not be able to detect any variance as it rolls across. Runout of .002" is acceptable for accurate loads. More than this and accuracy will suffer. Several manufacturers make runout gauges, I like the Forster system. The gauge is useful to identify steps in your ammo making process where runout is introduced.

Primary sources of runout include case sizing operations (pulling the expander ball up through the neck) and bullet seating (tilt) where each step will cause or add to a non-concentric combination. Variances in neck thickness in the same cartridge will cause runout too. That's why people who are interested in extreme accuracy turn necks to ensure uniformity.

Specialty dies are well worth the investment for reducing runout. The Forster and Redding seat dies with the tight tolerance sliding sleeve are excellent. Neck bushing dies have benefit as well.

Eliminating runout is what reloaders chase their tails over. Low runout takes a significant variable out of the shooting equation when striving for accuracy.