Case Grip on Bullets – Neck Bushing Size Is Just ONE FactorMany hand-loaders are unaware that the size of the neck bushing’s Inside Diameter (ID), is only one factor. Many factors can affect the grip of your bullet, and its ability release from the neck of the case. This article will teach you more about “case grip” and neck tension. We bet that you will gain knowledge to help you load more accurate ammunition with better ES/SD. Editor: This is a very important article. It is important that you read this article twice. The force required to release the bullet can have a significant impact on accuracy and ES/SD. You need to understand how different factors can affect the grip of a bullet.Neck tension (i.e. Grip on Bullets is a Complex PhenomenonWhile bushing size can be manipulated to alter neck tension (by using smaller or larger Inside Diameters), it’s not the only factor. It is important to know the many factors that can increase the resistance to bullet releases. Consider the overall “grip” of the brass on the bullet, rather than just the bushing size. Neck-wall thickness.2. The amount of bullet bearing surface in the neck (shank).
3. Surface condition of the neck (residual Carbon can act as an lubricant, ultrasonic cleaning will make necks “grabby”)
4. Neck length (e.g. 6mmBR neck vs. 6mm Dasher).
5. If the bullets are coated with an anti-friction material.
6. The springiness of brass (which is related with the degree of work-hardening, number of firings, etc.).
7. Bullet jacket material
8. The bullet’s outside diameter and whether or not it has a pressure-ridge.
9. The time between the bullet being seated and the firing of the gun (necks can stiffen over time).
10. How often is the brass annealed?
11. The length of the neck (e.g. You can only size half the neck.
12. Bullet “grip” may also change if you alter your seating depth, and it can even change if you ultrasonically clean your cases. Bullet “grip”, can also be affected by changing your seating depth or ultrasonically cleaning your cases. A dry or ultrasonically-cleaned case neck can have a high seating force. “In our Shooters’ Forum, a reader asked “How much neck pressure should I use?” This prompted an extensive Forum discussion where other Forum members suggested a specific number, based on their own experience, such a.001?,.002?? or.003??. These numbers are commonly used to represent the difference between the case-neck OD (after sizing) and the neck diameter of a loaded round with a bullet in place. The numbers represent the nominal amount (after sizing) of interference fit. etc.) Neck tension is a complex topic that can be useful for starting points. The amount of “grip”, or the force exerted on the bullet, is dependent on many factors. Neck-OD reduction is one of them. Understanding these factors will allow you to maintain constant neck tension over the course multiple reloadings.
You can try this simple experiment. You can do this simple experiment. The bearing surface (shanks) of the boat-tail bullet should be.150? Remove the bullet using an impact hammer. Next, take a second identical bullet and place it in a case with.300? Then, take another identical bullet and seat it with.300? You’ll notice that the bullets with a deeper seating are gripped harder.
I’ve also found that thinner necks – particularly the very thin ones used by many benchrest shooters of 6mm PPC – require more sizing in order to achieve an equivalent “grip”. Do your own experiment. Seat a bullet into a case that has been turned down to.008? Neckwall thickness and size down.003? Compare that to a case that has.014 thickness. Comparing the thickness of the neckwall and the sizing down to.0015?. You may find the bullet in thin necks pulls out more easily, even though it has supposedly more “neck tension”, when considering bushing size only. I think the guys using.0015 are probably right. “tension” on no turn brass may be closer to the guys who use.003 than either group realizes. “tension” may be closer to the guys using.003 than either group realizes.
We tend to describe the neck tension solely by bushing size as a convenient shortcut. When a man says “I run.002 tension”, he means that he uses a die/bushing to size the necks at.002? It is smaller than a loaded cartridge. We know a little about his neck OD after sizing, but can we be sure of how much force it takes to release the bullets? Maybe not… It is a bit simplistic to ask “should I load.001 tension, or.003 tension?” In reality, a.001? In reality, a.001? Reduction on a neck with a thin wall holding a bullet that has minimal bearing surface. We can easily verify and measure the ID of the bushing. We use bushing size to describe neck tension because it’s convenient and because other factors are difficult to quantify. Consistency and accuracy are the main goals. We want to find out the best neck tension that will give us accuracy and then maintain this amount of grip on bullet over time. You need to consider not only the bushing size but also how the brass has changed over time (work-hardened), and if other variables have changed (such as the level of carbon in your neck). The optimal neck tension can only be determined experimentally. You must test empirically in YOUR rifle with YOUR bullets, YOUR brass, to determine what works. You may need to adjust the nominal tension (i.e. Bushing size does not tell you the true “holding power” of the neck on a bullet or the energy needed to release the bullet. True bullet grip is more complex and is affected by many factors, some of them difficult to quantify.
