U.S. Army Marksmanship Unit added 2 new photos.
April 8 at 9:01am ·
(HL) Welcome to another weekly edition of the U.S. Army Marksmanship Unit’s “Handloading Hump-Day!” Our Handloading Shop staff are happy to share tips and techniques to help our fellow shooters obtain better accuracy with less effort, more precision and lower cost!
This week, we examine DETERMINING OPTIMUM CASE NECK TENSION FOR BEST 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. [Photo 1]
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, so don’t tune out just yet! 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.)
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 is where we usually start. Occasionally, 0.002" expansion works very well, so as always, keep an open mind!
Using the .260 Remington as an example, our loaded cartridge case necks measure 0.292”. [Photo 2] Simply subtract 0.003” from that, and use a bushing sizing necks to 0.289”. 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.
Note: this 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. In any event, we do not crimp rifle cartridges, and advise against it for accuracy handloads.
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, at extra cost.)
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.
There are companies which offer to convert one’s standard dies to accept neck bushings, and that gives much 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. It leaves fewer options than the bushing route, if one later changes case neck thickness.
As always, we hope this is helpful! Please feel free to share any useful information here that would benefit others. Until next week, be safe and good shooting!