My concentricity gauge shows me the expander ball is the culrpit at my house.

The benchrest guys expand with a mandrel from 220 Russian to 6mmppc with a mandrel.
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selm=2ks7kc%24p9u%40news.u.washington.edu

What is the difference that makes concentric necks with one and my lousy crooked necks with the other?

While I am not sure I understand that link I am not shy to jump in first it seems.

Looking at it from what I learned working in a deep draw house you have to start with as perfect metal as possible. This means that if the case necks are not uniform when you start they may even get worse. Just one of the variables that you can do something about is select a lot and make of case that has the most uniform neck wall thickness and least runout to start.

Then since you have a bench quality rifle I would turn the necks in a very precise way.

Next I would expand them a very little at a time with buttons that open the neck a little at a time. All this while use plenty of the best lubricant that you can get applied in an uniform manner. I would use a chassie grease with EP and moly.

The idea is to do every case the same. I am sure this is what you do anyway but it must be mentioned. Every little detail counts also such as a gradual and uniform inside neck chamfer.

There, I went first and invented something that has already been invented I bet.

I should have sent you over to Benchrest.com ?

Not sure if your question is regarding normal expanding after resizing, or expanding up a caliber as your illustration.

For the latter, I saw a neat gadget recommended in Handloader to eliminate expanding the hole off center when going up a caliber or two. It really grabbed me. It WILL work. Enough to cause me to communicate with Sinclair about turning one of their expanding mandrels for me. They liked the idea, and the main man was going to consider the tool for their regular inventory. They couldn't make one for me cost effectively so I'm doing it here. It's actually a stepped expander. The initial part is the exact inside diameter of the neck you are going to expand, and its exact length, or longer. This enables the mandrel to be centered in the shoulder and neck using the centered shoulder opening as a guide or pilot, before any expansion of the neck occurs. There is a 3� step up to the caliber for which you are expanding, which enables the mouth to expand gently while the mandrel is still centered on the shoulder opening below. I have much expanding to do imminently from 308 up to 338 for another 338WSM I'm building. This gadget should eliminate the off center necks that are common and which must be straightened with one or two fire formings before actual use for serious shooting.

As illustration, the neck ID of 300WSM cases is .305". That will be the OD of the initial part of the mandrel. For 223's that would probably be either .221" or .222". The WSM neck is approximately .300" long on new brass so that length would be minimum on the initial step on the mandrel, to insure the front of the mandrel is in the shoulder opening before any expansion of the mouth and neck commences.

If you are concerned with the expander AFTER resizing a case, the easiest way to insure concentricity is the use of bushing dies.

[ 03-07-2003, 00:31: Message edited by: Bob338 ]

99,
"Just one of the variables that you can do something about is select a lot and make of case that has the most uniform neck wall thickness and least runout to start."

If there is a run out of thickness around the neck, the expanding will make the thin part thinner and leave the thick alone?

338,
"It's actually a stepped expander"
You mean the neck is holding the expander concentric while the expansion is only going on in a small area ring?

Stepped like this Lyman M die?
http://beartoothbullets.com/tech_notes/archive_tech_notes.htm/39

[ 03-07-2003, 01:45: Message edited by: Clark ]

It's more of a garbage in more garbage out situation.

If a neck does not have uniform wall thickness for instance just pushing an unsupported button thru will only pull the heavy side more and create more runout and more variable neck tension for that matter.

In really deep drawing with length diameter ratios of 20/1 and more the metal going in must be flat to tenths across the blank. Such metal is run on Sendzimer mills that produce the most uniform strip.

Once you start drawing something without supporting both sides of the material it only gets worse or if we are lucky stays the same.

EXACTLY! Thanks for the link.