Many novice hand-loaders believe that neck tension is determined by the neck bushing’s Inside Diameter (ID). There are many factors that can affect the grip of your bullet and how it releases from the case neck. This article will explain neck tension and “case grip”. We are confident that you will gain the knowledge necessary to load more accurate ammunition with better ES/SD. Editor: This is an important article. It is worth reading twice. Variations in the force required for a bullet to be released can have a significant impact on accuracy and ES/SD. It is important to understand how many factors can alter the grip of a bullet. Neck Tension (i.e. Grip on Bullets is a Complex Phenomenon
Although we can control neck tension with tighter or looser bushings (with smaller and larger Inside Diameters), this is only one factor. Understanding the many factors that can increase or decrease resistance to bullet release is important. Instead of focusing on the bushing size (or even the internal neck diameter in full-length non-bushing sizing dies), think about overall brass-onbullet “grip”. Bullet grip can be affected by many factors, including: 1. Neck-wall thickness.
2. The amount of bullet bearing surface (shank), in the neck.
3. Surface condition of neck (residual Carbon can act as a Lubricant; ultrasonic Cleaning makes necks “grabby”.
4. Length of neck (e.g. 6mmBR neck vs. 6mm Dasher).
5. Whether the bullets are coated with anti-friction.
6. The brass’s springiness (which is related with work-hardening, firings, etc.
7. The bullet jacket material.
8. The bullet’s outside diameter and whether it has a pressure-ridge.
9. The time between firing and bullet seating can vary. Necks can become stiffer over time.
10. How often is brass annealed?
11. Size of neck (e.g. You can only size half of the neck.
12. The interior diameter of the bushing or neck section of a non-bushing die.
There are many other factors. It is important to realize that the bushing size does not define the end and beginning of neck tension questions. Because even though the bushing size remains constant, the bullet “grip”, depending on the condition of your brass, can change dramatically. You can alter the seating depth and bullet “grip” may change. It can even change if your cases are ultrasonically cleaned. John Whidden, 5-time U.S. National Long-Range Champion, adds that “our tests show that the condition of necks in terms of lubed or non, carbon inside or out, squeaky clean, or not” is more important than the bushing size. A clean, dry, ultrasonically cleaned neck makes for a very strong seating force. This was the prompt for a Forum discussion where other Forum members suggested a number based on their experience such as.002’s,.003’s, or.001’s. These numbers are commonly used to refer to the difference in case-neck OD after size and neck OD of loaded rounds, with bullet in place. The numbers are the nominal interference fit (after sizing). These “tension numbers”, which are commonly used (of.001’s,.002’s, etc.) can be useful as starting points. While these “tension numbers” (of.001”,.002’’ etc.) can be helpful as starting points for neck tension calculations, they are not the only ones that matter. The actual amount of “grip” on a bullet depends on many factors, including neck-OD reduction during size. These many factors will help you to maintain consistent neck tension over multiple reloadings. Grip on bullets can be affected by changes in seat depth
This simple experiment is possible. Place a boat-tail ball in your neck. With an impact hammer, remove the bullet. Next, take another identical bullet. Place it in a different sized case with.300″ of bearing surface (same size bushing/same tension). The deeper-seated bullet will be easier to grip. It is important to have a neck-wall thickness.
I’ve also found that thinner necks, especially those used by many PPC shooters require more sizing to provide equivalent “grip”. Do your own experiment. Place a bullet in a case with.008’s neckwall thickness and sized down to.003’s. Compare that to a case with.014’’ neckwall thickness and sized down by.0015’’. The bullet with a thin neck may pull out more easily, even though it has less “neck tension”, if you only consider the bushing size. Because thick necks are less elastic than thin necks, you may need tighter bushings when turning necks in order to maintain the same grip on the bullets (as opposed to no-turn brass). I believe the guys using.0015″ tension on no-turn brass might be closer to the guys using “tension” on turned necks of.003″ than either group realizes. A Better Definition of Neck Tension
We tend to describe neck tension using the bushing size as a quick shortcut. If a guy says that he runs.002 neck tension, it usually means that he uses a die/bushing to size the necks.002 smaller than a loaded cartridge. We know a little bit about his neck OD after he has sized it, but do we have any reliable information about the force required to get his bullets out? Perhaps not. The use of the term neck tension when we are referring to the neck diameter reduction by a die/bushing is a bit misleading. It is too simplistic to ask “Should I load with.001 tension?” When in reality, a thick neck may provide more grip on a deep-seated bullet than a.003’s reduction on a neck with very thin walls. We can easily measure and verify bushing ID. Because it is easy to measure and verify, we use bushing size to denote neck tension. These factors should not be ignored if you want consistent neck tension and maximum accuracy. This is what consistency and accuracy are all about, isn’t that? We want to find the optimal neck tension for accuracy and maintain that grip-on-bullet throughout time. To do this, you must look at not only your bushing size but also how your brass has changed over time (work-hardened), and whether any other variables (such the amount of carbon in your neck) have changed. The optimal neck tension must be determined experimentally. To find the optimal neck tension, you will need to test it empirically with your bullets and brass. You may also need to adjust the nominal tension setting (i.e. As your brass work-hardens, you may need to adjust the bushing size. The bushing size does not tell us everything about the neck’s “holding power” or the energy required to release a bullet. True bullet grip is a complex phenomenon that is affected by many factors, some of them very difficult to quantify.
