Neck Bushing Geometry — How Good Are YOUR 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 <.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 6x47 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 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″.
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- Neck Bushing Concentricity Tested — With Surprising Results!
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Tags: Busing, Die, Neck Sizing, Reloading
Hah, I was surprised to see my little post placed here
Still working on finding the cause. I no longer have any custom dies (sold two rifles, waiting on new ones). I’m pretty sure these custom dies were well within the same .0015″ as you were seeing. Maybe I’m just now noticing it on the factory Redding dies.
Yep, bushing dies induce runout, but I have proven to myself on the target that neck to body concentricity don’t mean squat.
Like my 20 dollar lee collet die.
Wouldn’t the squareness of the top of the die (the threaded piece that holds the bushing down) have a lot to do with how square the bushing sits in relation to the case under full load?
Yes, and that was the culprit. I reported the solution to my problem non my FB page where this story comes from, so maybe share it here as well:
I don’t use the decapping assy on the FL dies and I decap and tumble before sizing anyway. Without the decapping assy the locking nut is not there and the bushing can travel up, not sizing the neck. So I was using a Newlon stem and a washer to set the bushing at the desired height.
In hindsight: I sold the newlon die that the stem belonged to, so had to put back the original redding stem a week or so back.
After Jim’s kind not to check this, I measured the redding stem and found out it wobbles .004″, so this is indeed the cause of the runout I have. But because prior to this I had always used the newlon stem (.0005″ runout I just checked), I never gave it any thought as I never had issues with my 6×47 dies before. Hence I was baffled that my 6×47 now also shoowed a lot of runout.
So the cause of all this is the stem that holds the top of the bushing, and changing something that you don’t think twice about to cause issues like this.
Sorry for all the typos
One thing that I have discussed with Troy Newlon is that he takes the time to straighten the allthread that his decapping stems are made from. He is all about the details.
If you want straight bushings, my experience has been that the carbide bushings, are much better, due to the difference in how they are made.
The other issue steel is whether the hole is cocked in relation to the ends. You can ask Speedy about this.
Technique can also play a role. While sizing a case, if you pick up the bushing and tap the retainer with it very lightly, and reverse your stroke ever so slightly, that will allow the bushing to recenter for your up stroke.
A friend checked his two Rock Chuckers and found that the top of his rams were holding shell holders slightly off center. He modified shell holders and the top of the ram, on his lathe, and the concentricity numbers got noticeably better.
Another friend was using a lube pad that was designed for one kind of lube with another, and it was going on the cases unevenly. Switching to lubing with his fingers showed significant improvment.
I did an article on letting a FL (bushing) die float in the press threads, and for that combination, that made a significant difference. The point is that there are multiple sources and solutions.
If I got .001 runout on a case neck of a case that had been fired and was sized in a FL die without the bushing, based on experience, I would look beyond the bushing, and for that matter the die.
Does anyone know where to get larger carbide bushings? I am interested in .268 or so. Thanks!