October 7th, 2017

Reloading Tip: How to Set Optimal Case Neck Tension

USAMU handloading Neck Bushing Die tension springback interchangeable bushings

Each Wednesday, the U.S. Army Marksmanship Unit (USAMU) publishes a reloading “how-to” article on the USAMU Facebook page. A while back, the USAMU’s reloading gurus addressed a question frequently asked by prospective handloaders: “How much neck tension is optimal, and how should I select a neck bushing size?” The USAMU offers a straight-forward answer, suggesting that hand-loaders start with a neck bushing that sizes the neck so that it is .003″ less than the loaded outside diameter with bullet in place. From there, you can experiment with more or less tension, but this is a good starting point for many popular cartridge types.

USAMU Reloading

Determining Optimal Case-Neck Tension

This week, we examine determining the correct case neck tension for optimum accuracy. Our method is simple, but relies on the use of case sizing dies which accept interchangeable neck diameter bushings graduated in 0.001″ increments. (Those readers using fixed-diameter dies with expander balls aren’t forgotten, however. Methods of tailoring these dies for proper neck tension will be found below.)

In our experience across many calibers, sizing case necks 0.003″ under the loaded-case neck diameter usually yields excellent accuracy. In other words, the sized case neck expands 0.003″ when the bullet is seated.

USAMU handloading Neck Bushing Die tension springback interchangeable bushings

USAMU handloading Neck Bushing Die tension springback interchangeable bushingsBushing Choice for Optimal Sizing
Over the years, we have periodically experimented with increasing neck tension to possibly improve accuracy. In testing with machine rests at 300/600 yards, accuracy often deteriorated as neck tension increased; thus, 0.003″ expansion (from sized neck to loaded neck) is where we usually start.

Using the .260 Remington as an example, our loaded cartridge case necks measure 0.292”. Simply subtract 0.003” from that, and use a bushing that sizes necks to 0.289” (after springback). There are exceptions — sometimes, brass may be a bit soft or hard. Some case necks might need, say, 0.001” more tension, but in general, this works well.

This .003″ standard of neck tension works very well for single-loaded, long range cartridges. Depending on your caliber and firearm, it MAY also work very well for magazine-fed cartridges. If this neck tension proves inadequate for your purpose, one can increase neck tension as needed while monitoring for possible accuracy changes.

Special Considerations for Coated Bullets: If you are using moly-coated bullets, this significantly reduces the “grip” of the case neck on the bullet, and you can expect to have to tighten your case necks accordingly — particularly for magazine-fed ammunition. In any event, we do not crimp rifle cartridges, and advise against it for accuracy handloads.

Tips for Using Expander Balls
Many savvy handloaders avoid the use of expander balls in high-accuracy reloading, if possible. These can stretch cases and/or disturb the concentricity of the case neck vs. case body. If using a die with an expander ball, tapering both ends of the ball and polishing it to a mirror finish can significantly reduce these effects. (Special carbide expander ball/decapping stem sets are available for this as well.)

The typical dies used with expander balls are intended to take any cases the user may find, and size them down well below the ideal “spec” to ensure any cases will give good neck tension. The necks are then expanded up to provide heavy to medium neck tension as the expander ball exits the neck. The brass is over-worked, leading to premature work-hardening, and seated-bullet concentricity may suffer. However, the cartridges produced are perfectly adequate for most handloaders. Those who seek finest accuracy generally prefer not to over-work their brass if possible.

Another Option — Custom-Honed FL Dies
There are companies which offer to convert one’s standard dies to accept neck bushings, and that gives excellent flexibility. Another, more “old-school” approach, is to have the neck of one’s FL die honed out to the desired diameter for sizing, based on one’s case neck thickness. The expander ball may then be reduced until it barely touches the case necks after sizing, or it may be eliminated entirely. However, once performed, this modification is permanent and leaves fewer options than the bushing route, if one later changes case neck thickness.

Those shooters who turn their case necks for optimum neck wall thickness uniformity, or for a tight-neck chamber, will want to take the reduced neck wall thickness into consideration. For example, when setting up a 7mm match rifle to use a standard hunting die without an expander ball, the slightly thinner necks resulted in a perfect 0.003″ reduction in the fired-neck diameter. The result was a low-cost die that fit with custom precision and yielded excellent, match accuracy!

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