Reloading Basics - Case Neck Tension

When a bullet is seated into a case we expect it to grip sufficiently to remain securely in place until firing, at which point the projectile is released when the chamber pressure reaches the desired level. This grip, or ‘neck tension’has a big effect on the performance of your reloads and there are a number of factors to consider in order to get it right.

Tension numbers

Neck tension can be hard to quantify but one of the ways to express it is to use ‘tension numbers’ which, in simple terms, are the difference between the outside diameter of the neck of a sized case, and the diameter of a loaded round at the same place on the case. Figures of 0.001”, 0.002” or 0.003” are typical examples of tension numbers you might come across. The higher the number, then the higher the ‘grip’ the brass has on the bullet. This only quantifies the amount of tension applied by the seating/crimping die, and it does not reflect many of the other issues which influence neck tension.

Resizing

The amount of resizing done to cases will influence neck tension, as it dictates the diameter of the case mouth into which the bullet is seated. The amount of the neck that is resized can be varied, for example, only half of the neck can be resized, and this will make it less ‘grippy’.

The way in which the various sizing dies work is different and this also affects neck tension. Lee Precision neck sizing dies squeeze the case mouth onto a collet, while their full length resizing dies compress the whole case body back to size, and the effects are very different. For example, .308 Win PPU cases just will not grip certain bullets at all when they are neck sized, but when full length resized they grip perfectly and neck tension is ok.

Case-related issues

There are other factors which, although directly related to the bullet seating process, can cause the neck tension to vary, even when you are applying a consistent amount of pressure when seating/crimping.

The neck-wall thickness of your cases is the amount of brass that is being compressed by the crimping die. If you use different makes of case, and the brass thickness is different, then the amount of compression will be different. Thicker cases will be ‘squashed’ harder onto the bullet than thinner ones, and MUCH thinner ones might not engage in the crimper die at all and not be compressed.

The ‘springiness’ of your brass is also a factor. Very springy brass will crimp onto the bullet but as the pressure is released, the brass can spring back to a small degree, slackening off the neck tension.

Springiness will not just vary from brand to brand, factors like the age, the number of firings and a degree of work hardening of your cases will all come into play and, over time, reduce the springiness of your cases. If your cases are old and they do not seem to grip the bullets with sufficient tension, then it might be time to retire them. Heat-treating or ‘annealing’ does reverse the effects of work hardening and helps extend the life of your brass, but it is a subject in itself too involved to be covered in detail here.

Bullet-related issues

Bullet diameter is a critical factor affecting neck tension. An increase in bullet diameter, from say .308 to .309, will make the bullet a tighter fit in the case mouth and, if a crimp is applied, the pressure required to push the round into the crimp die will be increased, translating into potentially increased neck tension.

Bullet hardness should also be considered, as harder bullets will resist the pressure exerted by the case mouth as it is seated, while softer ones may yield more to accommodate that pressure.

There are a number of factors relating to bullet design that will affect the amount of neck tension. The area of bullet bearing surface (shank) in contact with the case, will directly affect the degree of grip and, as the amount of surface contact increases, so will the amount of tension. In very simple terms, the more of these surfaces there are holding onto each other, the more grip there will be. A bullet with a longer shank, of the same calibre, diameter and weight, will have more contact with the case, and a corresponding increase in chamber pressure will be necessary to overcome it.

Other variables

The grip a case has on the projectile will also depend on the condition of the surfaces in contact with each other. Residual case-sizing lubricant, case cleaning media, anti-friction coating on the bullets and the bullet jacket material, all affect the amount of ‘friction’ that can be achieved between the two surfaces. Molycoating is specifically designed to reduce friction between a bullet and the bore, but this ‘non-slip’coating also affects the way the case neck grips the bullet and has to be taken into consideration when loading your own ammo. Any substance on the surfaces in contact with each other will directly affect neck tension and chamber pressure.

Conclusion

Reloading involves a huge number of variables, most of which impact directly on safety (chamber pressure) and the consistency of your home-loads. Neck tension will have an influence on both of these issues and it is affected by a large number of factors including those considered above.

The key to regulating neck tension is consistency, using the same components, dies and materials throughout the reloading process. If you do change projectile, consider things like the bearing surface and the diameter, plus the likely effects of neck tension. Use the same brand of cases, sized in a consistent way, and keep them in batches that have all been fired the same number of times.