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