Thread: Concentricity

Based on some discussion in the "here and now" thread.

Concentric, at least the dictionary definition, means to have the same center.

So you could look at reloading concentricity as having inner and outer surfaces on a case neck spaced evenly apart - in other words having consistent neck thickness.

My understanding is that concentricity is more often used to talk about the neck circle and case body circle both having the same center, i.e. The neck not being skew or offset, also called runout.

In most chambers there is a small gap between the chamber wall and the outside surface of the case neck. Once you fire the bullet, the case expands fit tight in the chamber, and then springs back slightly (your "fireformed" neck diameter). With competition or custom chambers, that gap can be very small.

The fireformed neck diameter is often large enough to not grip a projectile, so you have to squeeze the neck down to a diameter small enough to hold the projectile, now we are starting to talk about neck tension.

Obviously if the neck is thicker on one side than the other, concentricity goes out the window, which is why some folks do neck turning. If the neck is an even thickness, the chances of consistent neck tension and minimal runout are higher. The type of sizing die you use can also affect concentricity. Dies that have a good reputation for minimising runout are bushing dies and especially collet dies.

@kiwi39 @Maca49 @veitnamcam interested to hear your comments and theories