These are widely considered a “step up” from a routine case sizing die. Well. Not always, or at least not in all ways. Lemmesplain.
First, a “bushing-style” sizing die is manufactured such that the user selects the desired bushing, and this allows control over case neck sizing. A good example is the Redding “S” die, and there are others.
By Glen Zediker
There are a few reasons neck bushing dies came about. Probably the main reason came from those who want to do away with the expander or sizing button. There’s a widely-held belief that an expander is a bad thing, that it contributes to non-concentric ammo. I disagree, with a couple of qualifications having been respected. One is that it’s been treated as I have said time and time again: polished and centered.
Anyone who has measured results from a conventional sizing die after the expander has been removed know that the case neck outside diameter is sized down way on smaller than needed to grip a bullet. That’s what the expander does coming back through the downsized neck: it opens it up to a workable diameter. The combination of the outside sizing and the inside sizing determine “case neck tension,” which is just the difference between the case neck diameters before and after bullet seating.
Here’s some math (and this is all about math): Let’s say loaded (with bullet) case neck outside diameter is 0.248 inches. If we want to attain 0.002 “tension” then sized case neck outside diameter needs to be 0.246. Although this varies with the alloy composition and the number of uses, every sizing operation done results in some amount of “spring back” from the brass. Brass is elastic. The most constant figure I can give is that this amount will be at least 0.001 inches. So. If we want a neck outside diameter to be 0.246 we need to use at the least a 0.245-size bushing. If a standard sizing die is reducing the neck outside to 0.240 (common enough), then it’s pretty clear the expander has a chore to open the neck back enough to net a 0.246 outside. That, pretty much, is the source of ills associated with expanders.
So. It looks like bushings are the answer. Well, not so simple as all that. There’s a few issues. The neck walls in a cartridge case get thicker after use and reuse. If there’s no expander to set the inside case neck diameter, then the inside case neck diameter is going to get smaller and smaller if only the outside of the neck is sized. Follow?
Another issue is that the neck bushing doesn’t or won’t size the full length of the case neck cylinder. There’s about a 0.020 area that doesn’t get touched. Whether that’s a problem or not depends on the bullet seating depth needed. Keeping in mind that there’s no expander functioning to open up the downsized neck cylinder, that little untouched area coincides with the location of the “dreaded donut” that can, and will, exist. This is a little elevated ring that lives right at the case neck, case shoulder junction. It’s daggone bad. More in another article. Upshot is that leaving this area untouched can contribute to its influence, and seating depth matters because if no portion of a bullet’s major diameter extends into this area, then it never factors. But most extend below it from a little to a lot.
If anyone chooses to use a bushing die, I strongly suggest that they still run an expander (or a mandrel as another step/station) in their sizing op. Inside sizing provides more consistent sizing results use to use to use than only outside sizing. If not, then there’s a lot to keep up with. Bushing size has to change as the necks get thicker, or the necks have to change, and that’s another two articles… Plus, some semi-autos leave ding-dents in the case mouth area, and an expander irons those out; outside-only sizing won’t.
I have a local machinist modify my conventional sizing dies by increasing the neck sizing area diameter. I get him to set a size that gives the expander 0.003 to displace. Mo math!