From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: WRT neck resizing
Organization: Hewlett-Packard Fort Collins Site

Doug White (gwhite@ll.mit.edu) wrote:

: If I use the same test case to check the seating die, I
: get loads that are more than 5 thousandths too long.

I get cartridge OAL variances that much myself even when bullet
seating pressure is down around 5 to 10 pounds, not the 50 to 70
pounds typical sizing dies enable.  The reason is probably not
so much press springing, but the bullet dimensions involved.

Most bullets vary close to 5 thousandths of an inch from their tip
back to a datum point equal to bore diameter just in front of their
bearing surface.  It is this datum point that determines how far
bullets jump from their seated position to the lands; not their tip.

If one does the measurements correctly with the right tools, one will
find out that some cartridges at the max limit of OAL have this bullet
datum point further back towards the case head than some cartridges at
the minimum OAL limit do.

And the distance between the ~.100-in. diameter contact point of the
seating stem to that bore diameter datum point further back on the
bullet also can vary several thousandths of an inch.

It is a common thing with traditional bullet seating to have the actual
jump-to-land distance spread greater than what the OAL spread is.  Two
methods keep the bullet's jump-to-land distance constant:

  * Use a seating stem whose bullet contact area touches the bullet just
    in front of the bore diameter datum point.

    This will work for several hundred rounds, then you have to back out
    the bullet seating stem a thousandth of an inch or so to compensate
    for throat wear.

  * Seat bullets out far enough so they press back into the case several
    thousandths of an inch as the bolt closes.

    You'll get zero jump this way and it's the most uniform and
    consistant there is.  The exception is for rapid fire and semiauto
    ammo, this is not a good idea; use the other method.

BB