Neck Sizing dies/collets

[Awf Hand]

Could someone explain to me how these things work and how do I select the right size for best accuracy.

Let me expand a bit:

I'm shooting cast lead bullets and unsure about what is the best neck tension on them. 

If I'm understanding correctly, a person measures the OD of a loaded round and calculates down a few thousanths for the right size.  The difficulty comes in when I'm shooting lead (GC) bullets with no crimp and I've found that a bit tighter neck works a bit better.  If I measure the OD I'll still get the same measurement, but I won't have a good handle on the "grip" of the mouth to the bullet.
I've posted this message in a couple of places. (setting tip-ups)

[jhalcott]

  I think you need an "M" die . this die expands and bells the case mouth to accept the cast bullet easier and without damage. They come in about any size from 22 to 45 caliber. After you size and reprime  the case you use this die. THEN you charge the case and seat the bullet using a seater die. The seater die SHOULD be adjusted to crimp the case at the very end of the stroke. Many cast bullet shooters do the crimping in a separate step to prevent damage to the brass from over long cases.
 A neck sizing die ONLY sizes the neck of a cartridge. These are used by shooters who are looking for 'BENCHREST" accuracy and do not want to set the shoulder of the case back. This is a CRUDE definition but I hope you get the idea.  Google "M" die and "neck die to see if they have any differences.

[Awf Hand]

I'm currently using the RCBS Cowboy ,mouth exp. die for 30-30Win shortened to work with my cartridges.  This does the same as the "M" Die and expands the full length of the neck, while slightly belling the case mouth.  I AM looking for BR accuracy and I'm NOT going to crimp.  The COL that I'm working in, would require the crimp to be in a non-cannelure portion of the bullet, which would cause deformation.   Crimping also requires the soft lead bullet to "uncrimp" the case as it passes through the reduced opening.  The projectile becomes slightly deformed as a result.  I have tried crimping and saw my groups open immediately with no other changes made.  This is fine for a "Thutty-Thutty" or BPCR where 1" plus groups at 100yds are tolerable.  (I'll get flames for that last sentence  )

What I'm hoping to determine is what size necks I should be creating to obtain and maintain the right amount of "hoop tension" on the bullets.  The brass has a certain range of elastic deformation before permanent deformation occurs, and I'm trying to work within that window.

I am going to order a VLD reamer next, and I may make a couple of steel bushings on the lathe just to find out what size works best.  The Redding catalogue recommends buying "1 or 2 sizes above and below the target diameter" for neck-sizing bushings.  Good for them.  They are 20$ each not including shipping.