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Old 07-13-2019, 11:09 AM
Pathfinder76 Pathfinder76 is online now
 
Join Date: May 2007
Posts: 15,779
Default Concentricity observations.

I've been loading off and on for the 7MM Mashburn since 2012. Love the round, but not being a SAAMI cartridge it has its issues. Reamers are not standardized and the selection of dies is extremely limited. RCBS, for years, has been the only one offering them (they still are the only ones offering form dies), but my late friend Bob Farese sent cartridges to Redding in 2012 and they are now offering dies. Both options are extremely expensive.

I own a set of the RCBS dies, form 300 WW Win Mag brass and have always had concentricity issues. Most of which comes from the seating process. I generally notice concentricity issues with seating any cartridge which baffles me to some extent. There are likely several factors.

With FL sizing, I have completely removed the expander ball stem and run cases into the die. They go in concentric and come out the same way. I have long believed that a non-bushing FL setup with no expander will produce cases with almost zero run out. Bushing dies produce some. Argue that as you wish. But we have chambers that are perfectly concentric cut with reamers. Dies are cut the same way. Generally, they are concentric. Bushing dies offer infinite flexibility in neck tension and are much cheaper than a set of varying custom dies with custom necks to accommodate case hardening and brass variance.

Now to achieve proper neck tension I run the cases over a Sinclair expander mandrel. Run out starts to appear. Usually 1.5 to 2 thou. I attributed this to neck wall issues now being forced to the outside. This morning I decided to experiment with the expander ball. I put it into the die, let the expander ball float by not tightening the lock ring at the top of the die (I've done this lots) and when retracting the case stopped with the expander ball in the neck. Then I tightened the lock ring. In theory, it should now be relatively straight. I then ran cases already sized the previous way into the die and suprise suprise I knocked almost a thou of runout off the case consistently. Now we are at 0.001 of perceptible runout. And I have reduced three steps to one. Bonus.

But the seating is the problem. Consistently adding 5 to 6 thou more runout. I use a Forster Co-Ax press and with this setup, and with the long throat of this rifle my seating die is almost too short. I have to back the die body out for breathing space. So I backed the seating stem all the way up and bottomed it out (RCBS seating stems are removed through the bottom of the die) tightened the lock nut and set the seating depth with the die body lock ring using a dummy. I then seated bullets. Zero runout. Instead of going from 1 to 5 or 6 thou, these cases have gone from 1 to 0. I don't know why, but I don't care. :-)

All this is likely not repeatable with another cartridge and die set, but if you work at it for 7 or 8 years you can maybe come up with a solution. In spite of all this I still have a custom set of Whidden dies on order.
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