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Old 07-14-2019, 08:51 AM
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Bushrat Bushrat is offline
 
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Quote:
Originally Posted by qwert View Post
IMHO, If you can consistently achieve .001” concentricity, with std RCBS dies, you are doing very well, (or experiencing possible measurement error).
I suspect you will do better with the Whiddens.

I suspect we agree on the importance of concentricity to avoid bullet yaw entering the throat/leede area and any possibility of the bullet swaging itself so the point is off bore center, (and fly in a ‘barrel roll’?, or at least in an unbalanced state?).

IMHE, case neck wall thickness variability can cause concentricity problems with most types of sizers. I suspect any die using an internal expander will cause any “neck wall issues now being forced to the outside”. Your concentricity gauge (IIRC a Sinclair with 4 balls and a vertical dial indicator?) may show this by indicating off the neck and then the side of the bullet adjacent to the neck.

IMHO, any neck wall thickness or hardness variability may cause the thin section of wall to yield sooner than thicker and stronger sections, which may cause the bullet base to yaw as it is released and starts into the throat.
Similarly, when being resized and expanded, the thin section will be worked more than the thicker sections, work harden at a different rate, and explain location of longitudinal neck fatigue cracks.

Many suggest that the combination of inconsistent wall thickness and expander friction may also tend to pull the neck out of concentricity. Some suggest carbide expanders might reduce this.

Some suggest that outside neck turning to uniform the wall is beneficial, and that turning an entire batch of brass to a consistent wall will result in greater average concentricity for the whole batch, and allow resizing with bushing only and no expander.
Others submit this can lead to formation of ‘donuts’ inside the neck / shoulder junction, and suggest that using a bushing that results in just a small amount of work to be completed by the expander ball.

I have 5 bushings for each bore caliber I load, (some think this is excessive, I like good tooling).

I suspect (hope?) that Collet Neck dies may slowly move brass from thick to thin areas of the neck wall, and are more tolerant of varying wall thickness, (YMMV).

You are experienced and knowledgeable enough to recognize you are at a high point of the bell curve, where small improvements become harder to achieve.
You also know that consistency is the proven route to precision.
I am confident you will achieve both.

Good Luck, YMMV.
Well said. Years ago I went to turning necks, have never looked back. Seating concentricity issues disappeared even with ordinary off the shelf standard dies. If one side of the neck is thicker than the other you will always be fighting it. Also if the case mouth is not perfectly square the bullet will contact the high side of the neck first when beginning to seat and tip the bullet out of alignment as it is seated. The neck even if perfectly straight and aligned will not straighten a tipped bullet or a bullet that is not aligned to go into the neck perfectly straight. If it enters the neck upon seating tipped or off center it will be crooked when fully seated. The brass neck is the weakest link and will deform to accommodate this misalignment, it doesn't have the strength to 'straighten' an already misaligned bullet as it is being seated.

Square case mouth with a proper concentric chamfer + equal neck thickness around its circumference + straight, centered and aligned bullet as it enters the case mouth = straight ammo.
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