Controlling Neck Tension

[crazywader]

Amazing level of knowledge on this forum! Lots of answers to questions posted so no need to post until now. Recently I've observed something that I'm unable to explain or control.

Started preparing some new brass for .223 WSSM (for bolt action). After FL resizing, I checked the neck inside diameter - they consistently measure 0.218" - or 0.006 less than the bullet to be seated (0.224"). The expander ball that originally shipped with the die (RCBS standard FL die) measured exactly 0.218.

Called RCBS and explained that I thought 0.006 was greater than what I wanted (would prefer .002-.003 of tension). They sent me a new expander ball that measured 0.224. My understanding is that brass will flex back after the expander ball passes through it so I expected to see an inside diameter of about 0.222 or slightly less with the new expander ball. I double checked to ensure when I pulled the expander ball through the neck it was AFTER the neck had already been sized by the die. Re-checked the inside diameter and it still measured exactly 0.218.

I understand that the die is supposed to control the tension (hence buying a die with a set of bushings) however, I don't understand why - if the expander ball is the last thing to touch the neck - why isn't the inside diameter wider than 0.218? I can't believe that the brass is flexing back that much - particularly so with the wider expander ball in place. (I have not yet turned the brass but plan to do so).

I prefer to understand what's going on before I go any further (buying a die with bushings, etc). Is there something I'm over looking, doing wrong, is the die the source, or is it something else?

Thanks,

Crazywader

 

[Tim Singleton]

What are you using to measure the inside diameter of your brass?
Most measure the OD of the brass and the bullet with a micrometer.
Then check again on a loaded round. To determine bullet grip

 

[BoydAllen]

You can play with expander mandrels that are designed for expanding prior to neck turning, until you find the right combination. They have less of an adverse effect on case straightness than do expander balls, when the ID of the neck part of a FL die is too small. What you are seeing is brass spring back. If you want to play with annealing, it could reduce the problem, but it would introduce an entirely new variable, and I am not sure that is what you are looking for. Since your brass is so concentric, it would not take much of a cut to clean it up by turning, and from there you could go to a bushing type FL die. Neck tenson is usually specified by the difference in the OD of a sized and loaded case neck, but this does not allow for the great differences in the force that it takes to move a seated bullet that result from neck thickness and degree of annealing. Based on these considerations you may want a "neck tension" (difference in diameters) that is very small, more like .001, at the maximum, and with that you will still probably have plenty of bullet pull.