Neck Tension 101 — It Ain’t As Simple as You Think
If you want to load ultra-accurate ammo and shoot very small groups, you should read this article, which we are re-publishing by popular demand. Many novice handloaders believe that neck bushing Inside Diameter (ID) size is the only important factor in neck tension. In fact, many different things will influence the grip on your bullet and its ability to release from the case neck. To learn the ins and outs of neck tension, take some time and read this article carefully.
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.
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. 6mmBR neck vs. 6mm Dasher).
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. Time duration between bullet seating and firing (necks can stiffen with time).
10. How often the brass is annealed
— and there are others…
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.
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.
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:
- Controlling Grip on Bullet — Why Bushing Size is Only One Factor
- 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
- Grip on Bullet — Many Factors Involved, Not Just Bushing Size
- The Complexities of Neck Tension — Why Bushing Size is Only One Factor to Consider
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Tags: Brass, Moly-coating, Neck Tension, Neck-Turning, Ultrasonic Cleaning
Neck tension is one of those things you need to decide ahead of time how crazy you want to get. Otherwise you can get carried away quickly. The more you learn the more you realize you dont know. I personally decided my time is better spent on the range. Though experimenting is fun in its own right.
The main reason so much confusion exists over tension is that the only information about it –is bad information,, and you’re perpetuating it right here. On your list affecting bullet grip:
#2 and #4 seated bearing and neck length mean nothing to grip where neck area is not springing back against or even touching bearing(like with partial sizing).
#3 and #5 surface conditions inside of neck/reduced friction means nothing to grip. These are factors affecting seating forces, which are independent of and different than grip.
#1 neck thickness and #10 annealing affects #6 spring back
#8 bullet diameter is the same as down sizing amount, which your article conceded as all but meaningless to actual grip(which is true for any amount beyond spring back)
#7 jacket material has nothing to do with grip
#9 is both true and false, as an unqualified bullet
That leaves ‘and there are others’ which is true and worth editor review before blurting.
Mike: Interesting comments. How did you come to your opinions? If you claim the article was “bad” information, then how did you get your “good” information? Do you have equipment to exactly measure the amount of frictional force required to pull a bullet out of the case? Do you have some sort of equipment to measure what happens to the case neck as the powder burns and tens of thousands of pounds of pressure are released? Have you conducted experiments with control groups to verify your conclusions? I am a bit of a science geek so I am very, very interested in the facts and methodology that support your statements. The extremely complex physics of what goes on during the micro-seconds of primer ignition to the bullet out of the barrel are virtually impossible to exactly know. So I was wondering if you were basing your statements on other articles you have read, personal experience, or actual scientific empirical data. Thanks for the clarification.
You cant rally do this subject without addressing concentricity, you can have uniform tension but I think concentricity will come into play and effect the results more than the uniformity of neck tension.
This topic always makes me think about crimping. I know most believe that it hurts accuracy but when I’ve tested crimps sometimes it helps. Granted most of the time in my tests it doesn’t but that sometimes is worth looking at. Sometimes.
Great article! Has anyone proven that a specific neck tention produces greater accuracy?
Im no world record holder, my experience is directly related to results on paper shooting prone from a prs type rifle so take this how you want. I could not put improved results on paper by monitoring neck tension. No matter how bad i wanted to feel like all my studying and experimenting gave me an edge. The most important thing i took away is that to little neck tension will leave the bullet in the lands if you try to unload a chambered round. I think theres more payoff getting each powder charge perfect.
Terry, I don’t known how etched in stone it is but generally light tension is preferred.
“but also at how your brass has changed (work-hardened) with time,”
just great– how do I do this?
One more point, if you tumble [with stainless media], try using a liquid car wash and wax on the final rinse of the brass. This will leave a small amount of residue inside the neck. I also dip the necks in the imperial graphite jar. This eliminates any grabby-ness.
why care what the neck OD is?.. would it not make more sense to measure the ID of the case, then it dont matter how thick the walls are, use a collet die like the LEE, resize to .003 or whatever less then the bullet dia. and be done? way less wear and tear on the brass over a bushing die
Neck tension is by far the toughest one, of the Holy Trinity of Precision Handloading, to crack and I’ve been working on it for nearly a year.
I agree with the factors mentioned in the article above. However, I would like to point out that focusing on the bushings to get the right neck tension is inherently an inaccurate path. Bushing affect the outside of the case neck and a .001 change in size has a big impact that causes seating pressure (measured by the hydro press) to jump by 20-30 pounds. So it my opinion that this is the wrong way to go about it. Its like a meat cleaver and what we need is a scalpel.
What really matters is not the outside diameter of the neck, it is the inside diameter that is critical to neck tension, that’s what touches the bullet.
If you think about it, it is easier to affect and control tension by expanding necks with a mandrel. I squeeze them down with a FL sizing die and then expand them with a mandrel. I can vary the size/diameter of a mandrel by .0001 thus giving me very specific amount of tension. I can go from 5-10 lbs range to the 15-25 lbs and systematically experiment to see which works best.
The trick is keeping it there because mandrels, like everything else, wear down with use. Even tool steel (D2) starts changing slightly after 800+ cases Another important factor is to make sure that the mandrel is perfectly straight and without “float”.
I have been working with a tool and die maker and I am confident I have finally cracked the third and last of the Holy Trinity.
There are ways to defeat each of the factors mentioned in the article but solving 3 out of 8 or 7 out of ten won’t work. They all have to be solved and assimilated in a process that uses the right tools and methods consistently, not an easy thing to do.