Optimize Neck Tension with Bushings, Expanders, and Annealing
by Sierra Bullets Ballistic Technician Paul Box
One thing that plays a major role in building an accuracy load is neck tension. I think a lot of reloaders pretty much take this for granted and don’t give that enough thought.
So, how much neck tension is enough?
Thru the years and shooting both a wide variety of calibers and burn rates of powder, I’ve had the best accuracy overall with .002″ of neck tension. Naturally you will run into a rifle now and then that will do its best with something different like .001″ or even .003″, but .002″ has worked very well for me. So how do we control the neck tension? Let’s take a look at that.
First of all, if you’re running a standard sizing die with an expander ball, just pull your decapping rod assembly out of your die and measure the expander ball. What I prefer is to have an expander ball that is .003″ smaller than bullet diameter. So for example in a .224 caliber, run an expander ball of .221″. This allows for .001″ spring back in in your brass after sizing, and still gives you .002″ in neck tension. If you want to take the expander ball down in diameter, just chuck up your decapping rod assembly in a drill and turn it down with some emery cloth. When you have the diameter you need, polish it with three ought or four ought steel wool. This will give it a mirror finish and less drag coming through your case neck after sizing.
Tips for Dies With Interchangeable Neck Bushings
If you’re using a bushing die, I measure across the neck of eight or ten loaded rounds, then take an average on these and go .003″ under that measurement. There are other methods to determine bushing size, but this system has worked well for me.
Proper Annealing Can Deliver More Uniform Neck Tension
Another thing I want to mention is annealing. When brass is the correct softness, it will take a “set” coming out of the sizing die far better than brass that has become to hard. When brass has been work hardened to a point, it will be more springy when it comes out of a sizing die and neck tension will vary. Have you ever noticed how some bullets seated harder than others? That is why.
Paying closer attention to neck tension will give you both better accuracy and more consistent groups.
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Tags: Annealer, Annealing, Expander Ball, Neck Bushing, Neck Sizing, Neck-Turning, Paul Box, Sierra Bullets
Last year a made a test, I prep 120 of a known good shooting load and I shot one 5 shots groups for accuracy and one 5 shots string for speed. Over a period of ( 1 year, every month ) my speed increased sd/es got lower numbers and the accuracy improved. I tried to keep the shooting condition close to the same all the time. My conclusion is that the only factor that would have change is the neck tension, The bullet would stick to the neck over a period and after 6/7 months no more speed increase was observe. I know this is contrary to most belief especially saying that the accuracy was improved over time, and I would suggest that you try your own test. Neck tension or retention if I could say it like that is not an absolute measure it will change over time,and each load is different depending on the condition they where first loaded at and how much the bullet will stick to the neck.We try to load the same as possible by measuring the size of the neck but in reality we should measure the force needed seat and/or pull the bullet.
Mike
Just as neck sizing bushings are available in different sizes, I’ve always wanted to purchase expander balls in specific sizes.
My dream is to combine both.
Imagine having control over both the amount of compression and expansion.
After all, the inside diameter is most important in the end, but without combining both, we live a life of struggle to get neck tension just right.
John Whidden (a multi national champion) said he found that case lubes that were lanolin based tended to dry and glue the bullet to the case neck over time. He prefers a popular aerosol spray lube that doesn’t tend to glue the bullets.
Mike, we have both bushings and expanders for our dies that do what you ask.
John Whidden
Tungsten Carbide expander balls remove the need for any lube and seem to also ‘iron-out’ any roughness inside the neck. They don’t wear as fast as the steel ones and they don’t “squeak”.
My major accuracy heresy is the use of the Lee ‘factory crimp” dies.
You can effectively applu “neck-tension” to any degree desired with these. they may shorten the life of the case, but, if you are annealing on a regular basis, this is not such a problem.
I’ve been using them since they were first introduced, starting with the .223 Rem. Initially the requirement was for a method of stopping bullets moving in the case every time a magazine rifle was fired. Then, I discovered that not only did the chronographed velocities SD tighten up, so did the groups.
YMMV.
There’s a mistake in this article where he recommends using an expander that is 0.003″ smaller than the bullet plus .001 spring back to get 0.002″ neck tension… Uhhh that math works out to 0.004″ of neck tension guys…
We’re talking about expander balls here… not sizing bushing… right?
If there is 0.001 spring back then you need 0.001 smaller than the bullet… not 0.003″
I called Redding once upon a time to ask for expander balls to run in conjunction with sizing bushings to refine neck tension even further… but the guy on the phone didn’t get it… I wish someone would.
I’d like to get expander balls in 0.0002″ increments so the bushing gets you close from the outside of the case and the expander ball tweaks it from the inside where it counts most.
I don’t think MH read the article correctly. If you squeeze the neck down 0.003″ and then the case springs back out 0.001″ after you stop squeezing it, the net reduction is in fact 0.002″ which gives you “neck tension” of 0.002″.
Mark C,
The article before getting to the bushing dies were you can set the diameter neck via interchangeable bushings for where you are correct..
It talks about standard dies which just oversized the neck smaller and then use an expander ball/mandrel to set neck tension. So the last operation to the hardening brass would be expansion so springback would be a contraction. So MH noted what I had.