Advanced Reloading — Checking the Geometry of Neck Bushings
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
- Neck Bushing Concentricity Tested — With Surprising Results!
- Neck Bushing Geometry — How Good Are YOUR Bushings?
- Custom-Honed Full-Length Dies — Better than Bushings?
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Tags: Bushing, Concentricity, Jim de Kort, Neck Bushing, Neck Dies, Redding Bushing
Thanks for the article.
I recently replaced a full length sizing die that varied .004 side to side.
I usually buy 3 bushings for each caliber I shoot. So should I buy three separate non bushing dies just to find the right one?
I send my Forster dies to Forster with a dummy round and they hone them out for .002 neck tension for $20.00. Then remove the expander as its not needed. I get no more runout than whats in the un-turned neck.
The way that I have successfully evaluated bushings is to simply use a neck micrometer, taking multiple readings around the bushing, measuring the thickness between ID and OD. If you have the budget, carbide bushings are ground rather than turned and bored, which results in much closer tolerances. You may also be pleasantly surprised to see that your necks come out at the correct diameter. In the past I had thought that the reason that I got sized neck ODs that were different than my bushings were marked was because of spring back from work hardening, that is until I tried my first carbide bushing. They are pricey, but if you want the best bushings they are.
I agree with Jim Borden. Also you should have some float in your die for the bushing. This will make the concentricity less important as your die will center on the neck. If the hole in your bushing is round and the bushing is perpendicular to its bore, you are good to go. Of course this assumes you have a good die.
More than likely heat treat fault.
Bushings were designed wrong from the start. John
Do you care to elaborate John?