Neck Bushing Concentricity Tested — With Surprising Results!
Do you use bushings to size your case-necks? Are you assuming that your bushings are actually round on the inside, with a hole that’s centered-up properly? Well you may be in for an unpleasant surprise, based on what our friend Jim de Kort recently discovered. Jim was concerned about the run-out on his brass. His cases went into his bushing-equipped FL die pretty straight, but came out of the die with up to .004″ run-out. “What gives?”, Jim wondered. “Could the problem be the bushings themselves?”
To answer that question, Jim decided to examine his bushings. Using an Accuracy One Wheel-drive concentricity gauge, Jim checked out some of his neck bushings. What he discovered may surprise you…
Neck Bushing Flaws Revealed
Trust no one… — Jim de Kort
Jim writes: “I measured the concentricity of my 6BR rounds today. I noticed they went into the neck-bushing equipped full-length sizing die with less than .001″ deviation but came out with .003-.004″. The culprit, it appears, was the bushing itself. Without it the cases stayed within .0005″ to .001″ deviation, so something was happening with the bushing.
One bushing had .00025″ deviation on the outside, yet almost .003″ on the inside, so it is crooked. But even when using a bushing that is within .001″ I still get .003″ runout after sizing. I repeated the same procedure for my 6×47 and got the same results. When using the bushing, concentricity suffers a lot.”
Before we bash the bushing-makers, we must acknowledge that many different things can contribute to excessive run-out and/or mis-alignment of case-necks. We don’t have all the answers here, and Jim would be the first to say that some mysteries remain. Still, these are interesting results that give all precision hand-loaders something to think about.
Jim Borden of Borden Accuracy also offers this tip: “Check the trueness of the face of the die cap. That has more to do with trueness than the bushing. Also check perpendicularity of hole in bushing to top surface. When I was making dies, the cap was made by threading and facing the threaded tenon in same setup.”
Editor’s Comment: Many people have great results with neck-bushing dies, but Jim isn’t the only fellow who has seen some very odd results. I personally employ honed, non-bushing dies for many of my chamberings. These non-bushing dies (with the necks honed for .002-.003″ neck tension) produce extremely straight ammo, with run-out consistently under .0015″.
Similar Posts:
- Checking the Uniformity of Neck Bushings — Some Surprises
- Uniformity of Die Neck Bushings — Run-out Revealed
- Advanced Reloading — Checking the Geometry of Neck Bushings
- Neck Bushing Geometry — How Good Are YOUR Bushings?
- Choosing the Best Neck Bushing Size — Tips from John Whidden
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Tags: Bushing Concentricity, Jim DeKort, Neck Bushing, Neck Sizing, Sizing Bushings
Once you have decided what bushing your are going to use for a particular application, if you really want a better bushing, I suggest that you try the more expensive option of carbide. They are ground rather than turned and much more dimensionally correct. Also, I have found no advantage to coated bushings. I prefer the Wilson brand because you can get a half thousandth more sizing by flipping them over. Whidden bought out his bushing subcontractor and those bushings are well made.
Great info Boyd. Thanks for sharing this valuable piece of info.
Paul
I’ve found that the less you have to reduce the neck diameter, the more concentric the necks are.
That builds a case for reduced clearance necks in the chambers.
The more you size them down, the more they can get off center.
Another key point when using bushing dies… Ever notice how easy it is to retract the press after sizing the neck only die?
That’s because these bushings induce a taper if sizing down more than about 0.008” and the mouth is about 0.002” smaller than it is closer to the shoulder.
My work around is to size the necks twice… the first pass is 0.002” larger than my target bushing… The second pass with the final bushing size makes the neck walls parallel.
i get run-out from my redding body die
but on testing concentricity with concentricity gauge; maybe this method is wrong.
imho, it doesn’t matter if inside-to-outside of the bushing is not concentric. on outside bushing is floating and it can be OK with center of the neck.
like j.borden say: check perpendicularity of hole in bushing to top surface.
i think that it doesnt matter if outside of bushing is oval; inside must be perfect round AND this hole MUST be perpendicularity to the case.
with concentricity gauge you cant measure that; imho…