Is your neck tension set correctly or do you need help finding the right bushing/expander to use? Enter your bushing diameter, measured case neck wall thickness, and bullet diameter below to see if you are using an appropriate size. Or, enter your caliber and case wall thickness to see which bushing or expander size you may need. Dimensions in thousandths of an inch.
Neck Tension Calculator
Bushing diameter • neck wall thickness • spring-back
| Target | 0.001″ | 0.002″ | 0.003″ |
|---|---|---|---|
| Bushing Diameter | — | — | — |
| Expander / Mandrel | — | — | — |
Bushing diameter sizes the neck outside diameter and depends on wall thickness. Expander/mandrel diameter sets the neck inside diameter directly and doesn’t depend on wall thickness — use whichever matches your die setup. Both figures are corrected for the spring-back entered above.
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Understanding Neck Tension
Neck tension is the interference fit between your sized case neck and the bullet seated inside it. As an engineer this term has always bugged me because tension is a force causing stretching. This isn’t a force measurement, it’s just the difference in bullet diameter and case neck inside diameter. However, the interference does cause a force that holds the bullet in place. Too little force, and the bullet is loose and can shift position. Too much, and the effort to seat the bullet in the case can cause damage or deformation. Consistent and even force on the bullet leads to consistent release when fired, and consistently is accuracy.
This is only if using neck sizing bushings. If using any expander ball or mandrel, that will determine final neck tension. In that case, use an expander that is .001-.003 below bullet diameter. There will be some spring back with that as well.
Watch: Full Length vs. Bushing Sizing Dies — Intro to Neck Tension
How the Numbers Work
A bushing sizes the case neck's outside diameter, not the inside diameter directly. To find the resulting inside diameter, subtract twice your measured neck wall thickness from the bushing diameter (since the wall exists on both sides of the neck). Brass also springs back slightly after the sizing ball or mandrel is removed, opening the neck ID back up by a small, alloy-dependent amount — typically 0.0003" to 0.0008" on modern brass, though this varies by manufacturer, anneal state, and thickness. Neck tension is then simply your bullet's diameter minus that spring-back-corrected inside diameter.
NEED BUSHINGS AND DIES?
- RCBS Matchmaster bushing die set
Spring-back varies by brass alloy, hardness, and anneal cycle — measure your own fired-and-resized necks with a ball micrometer when possible for the most accurate figure.
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