The Complexities of Neck Tension — Why Bushing Size is Only One Factor to Consider
In our Shooters’ Forum a reader recently asked: “How much neck tension should I use?” This prompted a Forum discussion in which other Forum members recommended a specific number based on their experience, such as .001″, .002″, or .003″. These numbers, as commonly used, correspond to the difference between case-neck OD after sizing and the neck OD of a loaded round, with bullet in place. In other words, the numbers refer to the nominal amount of interference fit (after sizing).
While these commonly-used “tension numbers” (of .001″, .002″ etc.) can be useful as starting points, neck tension is actually a fairly complex subject. The actual amount of “grip” on the bullet is a function of many factors, of which neck-OD reduction during sizing is just one. Understanding these many factors will help you maintain consistent neck tension as your brass “evolves” over the course of multiple reloadings.
Neck Tension (i.e. Grip on Bullets) Is a Complex Phenomenon
While we certainly have considerable control over neck tension by using tighter or looser bushings (with smaller or bigger Inside Diameters), bushing size is only one factor at work. It’s important to understand the multiple factors that can increase or decrease the resistance to bullet release. Think in terms of overall brass-on-bullet “grip” instead of just bushing size.
One needs to understand that bushing size isn’t the beginning and end of neck tension questions, because, even if bushing size is held constant, the amount of bullet “grip” can change dramatically as the condition of your brass changes. Bullet “grip” can also change if you alter your seating depth significantly, and it can even change if you ultrasonically clean your cases.
Bullet grip is affected by many things, such as:
- 1. Neck-wall thickness.
- 2. Amount of bearing surface (shank) in the neck.
- 3. Surface condition inside of neck (residual carbon can act as a lubricant; ultrasonic cleaning makes necks “grabby”).
- 4. Length of neck (e.g. 6BR neck vs. 6BRX).
- 5. Whether or not the bullets have an anti-friction coating.
- 6. The springiness of the brass (which is related to degree of work-hardening; number of firings etc.)
- 7. The bullet jacket material.
- 8. The outside diameter of the bullet and whether it has a pressure ridge.
- 9. The time duration between bullet seating and actual firing (necks can stiffen with time).
- 10. How often the brass is annealed
— and there are others…
Seating Depth Changes Can Increase or Decrease Grip on Bullet
You can do this simple experiment. Seat a boat-tail bullet in your sized neck with .150″ of bearing surface (shank) in the neck. Now remove the bullet with an impact hammer. Next, take another identical bullet and seat it with .300″ of bearing surface in another sized case (same bushing size/same nominal tension). You’ll find the deeper-seated bullet is gripped much harder.
Neck-Wall Thickness is Important Too
I have also found that thinner necks, particularly the very thin necks used by many PPC shooters, require more sizing to give equivalent “grip”. Again, do your own experiment. Seat a bullet in a case turned to .008″ neckwall thickness and sized down .003″. Now compare that to a case with .014″ neckwall thickness and sized down .0015″. You may find that the bullet in the thin necks actually pulls out easier, though it supposedly has more “neck tension”, if one were to consider bushing size alone.
In practical terms, because thick necks are less elastic than very thin necks, when you turn necks you may need to run tighter bushings to maintain the same amount of actual grip on the bullets (as compared to no-turn brass). Consequently, I suspect the guys using .0015″ “tension” on no-turn brass may be a lot closer to the guys using .003″ “tension” on turned necks than either group may realize.
Toward a Better Definition of Neck Tension
As a convenient short-cut, we tend to describe neck tension by bushing size alone. When a guy says, “I run .002 neck tension”, that normally means he is using a die/bushing that sizes the necks .002″ smaller than a loaded round. Well we know something about his post-sizing neck OD, but do we really have a reliable idea about how much force is required to release his bullets? Maybe not… This use of the term “neck tension” when we are really only describing the amount of neck diameter reduction with a die/bushing is really kind of incomplete.
My point here is that it is overly simplistic to ask, “should I load with .001 tension or .003?” In reality, an .001″ reduction (after springback) on a thick neck might provide MORE “grip” on a deep-seated bullet than an .003″ reduction on a very thin-walled neck holding a bullet with minimal bearing surface in the neck. Bushing ID is something we can easily measure and verify. We use bushing size as a descriptor of neck tension because it is convenient and because the other important factors are hard to quantify. But those factors shouldn’t be ignored if you want to maintain consistent neck tension for optimal accuracy.
Consistency and accuracy — that’s really what this all about isn’t it? We want to find the best neck tension for accuracy, and then maintain that amount of grip-on-bullet over time. To do that you need to look not only at your bushing size, but also at how your brass has changed (work-hardened) with time, and whether other variables (such as the amount of carbon in the neck) have changed. Ultimately, optimal neck tension must be ascertained experimentally. You have to go out and test empirically to see what works, in YOUR rifle, with YOUR bullets and YOUR brass. And you may have to change the nominal tension setting (i.e. bushing size) as your brass work-hardens or IF YOU CHANGE SEATING DEPTHS.
Remember that bushing size alone does not tell us all we need to know about the neck’s true “holding power” on a bullet, or the energy required for bullet release. True bullet grip is a more complicated phenomenon, one that is affected by numerous factors, some of which are very hard to quantify.
Similar Posts:
- Case Grip on Bullet — There’s More to Neck Tension Than Just Bushing Size
- Case Grip on Bullets — Neck Bushing Size is Just ONE Factor
- Controlling Grip on Bullet — Why Bushing Size is Only One Factor
- Grip on Bullet — Many Factors Involved, Not Just Bushing Size
- TECH TIP–Neck Tension vs. Time
Share the post "The Complexities of Neck Tension — Why Bushing Size is Only One Factor to Consider"
Tags: Annealing, Brass, Bushing, Cartridge, Neck Tension, Redding
Maybe if someone drilled and tapped a bullet, from the nose toward the base, it could be used to do bullet pull experiments by screwing in a small screw eye after seating. I would think that one of the new all gilding metal Hornadys would be ideal, giving better mechanical properties, for this application,than a lead core.
they do make tools that measure the amount of force required to seat the bullet… from there, we could just trust good ole Newton.
tom
One thing that was missing was whether your brass was made for that cartridge (lets say “260 Remington”) and you used “243 Win” or “7mm-08″ brass. In my (limited) experience the 243 brass maintains a tight grip (more spring-back) for several firings.
While there are tools for comparing seating force(I use one), seating force and bullet grip(tension) are two completely different things.
Both are important, but not for the same reasons.
Seating force affects resultant OgvOAL on seating.
Tension affects tune.
There is currently no tool available to measure tension.
First – thanks for publishing discussions such as this. This helps us all to improve and become more aware.
The later statement “When a guy says, ‘I run .002 neck tension’, it’s safe to assume he is using a die/bushing that sizes the necks .002″ smaller than a loaded round.” is a much better description.
Editor: Josh … I think I understood your point. For clarity, I modified the first paragraph to explain that the .001, .002, .003 numbers folks toss around normally refer to the difference between a sized neck OD and the OD with the bullet in place.
These are what people mean Josh, but none of it’s actually true.
A sized neck OD .002 under loaded OD amounts to the INTERFERENCE FIT, and does not directly mean anything about tension.
Over-anneal a neck & size it down(only), you’ll find that it is left at a smaller OD(because it springs back less from sizing). This will also cause an increase in seating force, but tension itself is lower with that annealed neck. You can probably pull the bullet with your fingers..
I try to prepare my brass (Lapua) as meticulously as possible, but I measure my neck tension in a very expensive way.
Quickly here is my preparation technique (.308 Winchester)
1) Anneal (even new Lapua)
2) Full length resize with a .3090 ball
3) Ream inside of neck with Forster reamer.
4) Neck size to ID of .3050
5) Trim outside neck (sorry, I don’t remember my exact wall thickness measurement off the top of my head). Minimal trim and consistent. Now comes the expensive part.
50 rounds is enough for about anything I shoot, so I meticulously prepare 100 rounds and put them in a 100 round bullet case.
I then shoot them at nothing through my chronograph, recording the MV of each. I then go home and put all the MVs in an Excel spreadsheet and set aside cells to record mean, median, mode, max, min, ES and SD.
I pull cases from the max and min alternately and recalculate until my ES is <= 20 (usually resulting in an SD of <= 6). I declare all the rest of the cases usable.
You'll probably cringe at this, but I then go ahead and anneal those usable cases and run them through my Thumler Tumbler. I neck size and reload. Doesn't seem to matter, they almost invariably result in an ES of 20 or less on the next firing.
It hurts me to fire a round without purpose and put 100 rounds through my barrel… but what works (for me) works and so far I'm willing to pay the price for the vertical consistency.
If someone could only teach me to read the winds on Range 4 at Quantico from the 1,000 yd line, I'd be the champ.
Good article. I was surprised that the Lee Collet Die wasn’t mentioned in this process as an alternative to bushing dies because of the recent praise of this die on a post on this site by quite a few people.