Written 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 give 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 thru your case neck after sizing.
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.
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.