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K&M Mandrel Causing Run-Out...?

Experimenting on some Peterson 6mmBR cases that have 3 firings on in my rifle.

Process: Brass gets FL sized in an LE Wilson die with a .265” bushing. Fire formed brass measures less than .001” run out both inside and outside the necks. FL sized brass is usually under .0025” both inside and outside the necks.

Fed through a K&M expander marked for .002” tension, my run out goes up significantly; anywhere from .003” to .005” inside the necks.

Ive tried both case lube and Redding dry neck lube while expanding. Tried leaving the mandrel loose in the press as well as locking it down. Spinning the case as it meets the chamfer of the mandrel. Expanding, rotating and re-expanding... I’m at a loss.

My last culprit other than the mandrel/holder is the press itself. I’m using an original RCBS JR from the 80’s, with about 18 years service to me. It’s plenty sloppy, but part of me doesn’t want to blame the press if leaving the die loose to induce slop should promote self centering.

Any suggestions on keeping my run-out in check during expanding?
 
Paul,

Im expanding a total of .002” with the mandrel.

I’m going to clean and anneal the whole batch, run them back through the FL die and mandrel them up again and see if they turn out any straighter.

I also read that the shell holder with the spring steel clip could be influential on the case head a bit. Going to be converting that over to o-ring retention before they go through the process again as well.
 
Seems like the brass is telling you that it doesn't like to move uniformly that much. Annealed Brass? Have you tried a smaller mandrel.
 
If you are moving the brass out .002 with the mandrel, try sizing the neck with a 266 bushing. That way you won't be moving the brass more than you need to.
 
Reporting back with much better results...

Cleaned brass, annealed, FL sized @.265”

Removed shell holder and spring clip, cleaned face of ram and the shell holder. Left the spring clip OUT entirely.

Mandrel body is about 1/2 turn past contact on an o-ring. Mandrel is tight in its body.

Spun the shell holder and case as the mandrel enters the neck under the weight of the handle. Expanded the necks in one motion and gave it a ~3 second dwell time. Redding dry lube used.

Run out measured inside the necks at .001” - .0035”
 
You might try using a Lee Collet Die after FL sizing with your Wilson bushing die. The collet has fingers that force the neck against the mandrel providing a kind of self-centering effect.
 
I was gonna suggest looking at the shell holder as well.

One thing to try,simple enough. With no die in press,put a case in shellholder and bring it up high enough to where you can sloooowly turn shell holder,and/or case. Look for off centered behavior. Not definitive,but can occasionally show up a problem with case head,shell holder,or some abnormal press issue?
 

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