Rifle round concentricity epifany

[TBF]

A famous gunwriter once said,

"If you are experiencing concentricity problems, remove the expander and size a case. If the runout is gone, the expander is to blame. "

This is not really accurate.

If you factor in the neck walls not being perfect, the hole the bullet goes in is not in the exact same center as the outside of the neck.

D'oh ! ( just like Homer Simpson )

A real forhead slapper ( for me ), and wht I have a pink spot on my forehead.

After measuring stuff I don't really have time for, I now see the error of my ways.

If the round is sized in the die without the expander, the case and neck will be aligned with each other. ( DUH ! ) This is what the concentricity guage measures when you use it on the neck, the misalignment of the neck to the body.

When you put the expander back in and size the same case, the amount of runout will increase related to the difference between the thick side and the thin side of the neck. The expander aligns ( more or less ) the hole the bullet goes into with the center of the case.

Measuring the outside of the case for runout only shows the difference between the inside and outside, plus ( or minus depending on the phase of the moon ) the actual runout induced by the sizing operation.

So simple, it escaped me for almost 20 years.

D'oh !

Measuring the wrong thing is counterproductive, the hole in the middle is the important part.

Travis F.

[AikiDale]

This is chapter 11 of the Tao Te Ching from a public domain translation:

11

Thirty spokes are joined together in a wheel,

but it is the center hole

that allows the wheel to function.

We mold clay into a pot,

but it is the emptiness inside

that makes the vessel useful.

We fashion wood for a house,

but it is the emptiness inside

that makes it livable.

We work with the substantial,

but the emptiness is what we use.

2500 years of technology later and it's the same old thing!

[TBF]

D'oh !

2500 years, dang it !

Oh well, you learn something new every day, sometimes whether you want to or not.

I'm finding that the more I know , the more I don't know as much about what I thought I knew...

Darn measuring tools are the root of all time consumption.

I am liking the .200 groups though....

Travis F.