Basics: Neck Tension

Neck Tension, Bushing Dies and Other Reloading Mysteries
by Germán A. Salazar

Neck tension is a frequently discussed topic among handloaders as you step past the basic level - and for good reason, neck tension is an important element in producing the most accurate loads possible and keeping your brass functional for a longer time. 

Fundamentally, neck tension is a term used to describe the interference fit between the inside diameter of the neck and the outside diameter of the bullet.  In short, we're going to seat a bullet into a neck that is smaller than the bullet and this can only be accomplished because the brass will yield under pressure and the inside diameter of the case neck will open to accept the bullet.  However, because brass is a fairly live and springy metal, the neck tries to return to its former size, thus applying tension to the bullet.  While neck tension can be measured in units of force necessary to push the bullet in, it is more common to describe it in the linear dimension terms of neck's outside diameter before and after seating the bullet.

As I just said, we use the neck's outside diameter as our basic reference despite the fact that it is the inside diameter that we are actually interested in.  That's because most reloaders don't have a set of gauge pins or tiny inside micrometers to take direct measurements of the inside diameter of case necks, accordingly, the outside diameter serves as a useful proxy as long as we know how thick the necks are.  Calipers cannot be used to measure inside diameter (a common question from reloaders) because the blades are not in line with each other so you are not measuring a true diameter.  Neither can calipers be used to measure neck thickness with any degree of precision because there are slight irregularities in the neck, especially at the mouth, that will affect the reading.


A tubing micrometer, which has a fixed anvil with a ball tip is the most accurate tool for measuring neck thickness.  As you'll note in the picture above, I've created a small step for the case to rest on, this ensures that I'm measuring all necks at the same distance from the case mouth.  While I could have gotten by without the step for .30-06 cases such as that in the picture, other cartridges with shorter necks couldn't be measured consistently without it.  The step is simply a small piece of leather taped to the micrometer to elevate the case a bit, nothing fancy.

As the micrometer indicates, this piece of brass is 0.0125" thick at the point measured.  That is the dimension to which I normally turn my .30-06 brass.  When turning, I measure each piece of brass at three locations along its circumference, but that's not relevant here.  If your brass isn't neck turned, take a few measurements on a few piece of brass in order to get a solid idea of average neck thickness.  You will find some inconsistencies, and that's life with unturned brass, just be aware of them and determine the average thickness.


A conventional sizing die establishes neck tension regardless of neck thickness by simply sizing the neck down more than it needs to be and then expanding it from the inside with the expander.  While this works well for general reloading purposes, it tends to overwork the brass leading to early neck splits and it doesn't allow you to fine tune your loads by varying neck tension.  Redding, Hornady, Forster and perhaps others make sizing dies with interchangeable bushings to allow you to size the necks down just enough for your particular lot of brass and desired neck tension.  The cutaway die shown above has an expander in place, but for most bolt-action use, the expander is removed and replaced with a simple threaded retainer for the decapping pin.

The cutaway die gives you a good look at how the bushing (the gold colored piece) is held in the die by the top plug.  While you might see or hear references to the bushing "floating" in reality, it is held in a close tolerance hole and is not free to move.  In the Redding Competition Neck Die with the micrometer top, the bushing can be allowed to move upward, thus limiting how far down the neck it sizes, but there is no practical reason for a Highpower shooter to do this.  The Type S dies, like the one shown here, don't have that level of adjustment - and that's no loss.

The pointer in the photo above is indicating the junction of the bushing with the shoulder area of the die.  You will notice in your sized cases that there is a slight bulge right at this point.  That's because the bushing has a small bevel to allow the neck to enter without shaving brass, so the lowest portion of the neck which comes to rest in that bevel doesn't get sized as much.  The bulge will have no negative effect on accuracy or brass life.  However, if you plan to neck turn, then the cases should be sized with a conventional non-bushing die prior to the turning operation.  That's because the neck turner would cut the bulge off, thus dangerously weakening the necks at that point and perhaps leading to a neck separation upon firing.

Well, with that general discussion of dies and brass thickness out of the way, let's talk about how to set neck tension to your needs.

For the sake of having an example, we'll use a Lake City .30-06 case that has been neck turned to 0.0125" as shown above.  Let's do some quick math - if the neck wall thickness is 0.0125" then the total of both sides is 0.0125" x 2 = 0.0250".  Assuming that our bullet is 0.308" in diameter, then the loaded cartridge should have an outside neck diameter of 0.308" + 0.025" = 0.333".  So, if our final diameter will (hopefully) be 0.333" and we want to have 0.002" neck tension (a pretty standard amount), then we'll need to size our neck to 0.331" before seating the bullet.


So, we have our fired case and are ready to size it.  Why don't we just run it through a die with a 0.331" bushing and be done with it?  We've already determined that's the desired outside neck diameter before seating the bullet, right?  Well, in some cases, but not most, you can do that.  In order to make that determination, however, we need to do some measuring and checking.  As you can see above, the outside diameter of the fired case neck is 0.339".  Since it was fired in a chamber with a SAAMI standard chamber with a chamber neck diameter of 0.340", and brass tends to spring back a little after firing, that's exactly what we would expect to see.

Now for another bit of math - we need to take our case neck from 0.339" down to 0.331", a reduction of 0.008" in outside diameter; this is a fairly typical amount.  What I have found over the years, as have many other reloaders, is that when you use the bushing dies to size down more than 0.005" at a time, they really don't do a very good job.  The result tends to be poor concentricity of the sized neck to the body and an outside diameter that is smaller than the marked bushing size. In fact, the necks might tend to be somewhat funnel-shaped, leading to some real interesting neck tension situations.

The best approach is to limit your sizing to steps of no more than 0.005" each and to make multiple passes as needed - I've never seen a situation that required more than two passes, so don't panic.


The first step in my resizing process for this brass was to size it with a full-length Redding Type S bushing die with a 0.334" bushing.  That brought the neck's outside diameter to 0.334", just as the bushing is marked, and set the headspace to my specifications for this rifle (see the article on setting headspace if needed).


The next step is to size the brass in a Redding neck sizing die with a 0.331" bushing, and, as shown above, the outside neck diameter is indeed 0.331" after this step.  Had we gone straight to the 0.331" bushing, this wouldn't be the case at all.  While I use a neck die for the second pass, you could use the same full-length die as in the first pass, although you should be careful to ensure that you aren't setting the shoulder back more on the second pass.  In reality, it's a lot easier to switch to a neck die for the second pass.  UPDATE: I have found that concentricity is slightly better if the first pass is with the neck die and the second pass with the full-length die.  Click here to see article on two-steo sizing with bushing dies.

As I mentioned before, brass is springy and these bushings take that into account.  However, as brass hardens a bit after repeated firing/sizing cycles, it may lose some spring and you may have to use a different bushing to get back to the desired outside diameter.  Don't worry, trust your measurements and give the brass what it needs in order to end up at the dimension you need.


Now that the neck is properly sized, let's have a look at the bullet.  Actually, this should have been done when we were measuring neck wall thickness before doing all the math, but since Sierra is pretty consistent, I left it until now.  In reality, there are better ways of measuring bullet diameter than calipers, but this isn't terribly critical, a few ten-thousandths difference here isn't what we're worried about, we want to make sure this is a 0.308" bullet, not a 0.309" like the Lapua D46, for instance (if it were a 0.309" we would have selected a different final bushing).  So, as the photo shows, this is a 0.308" bullet and our math is probably correct.


The payoff: here's the outside diameter of the finished cartridge and it is, of course, 0.333", meaning we have 0.002" neck tension because the finish sized case neck measured 0.331" before seating the bullet.  Now, we can use different final bushings to experiment with varying levels of neck tension and attempt to gauge its effect on accuracy.  If you're shooting a well chambered bolt action rifle and don't care to experiment too much, 0.002" neck tension will be a good place to set up your dies.

As a final note, I happen to know that the chamber neck diameter in the rifle is 0.340" since I own the reamer and have the print (yes, there can be tiny variance on that number).  However, the fact that the fired brass measured 0.339" is also a very good indicator, since 0.001" springback is about right in this size range.  So, with a 0.340" neck and a 0.333" loaded round, we have 0.007" clearance, a very safe amount for a Highpower rifle that gets used in very dusty conditions.  While we can get away with a bit less clearance, there's nothing wrong with this much for our use, brass will last through a lot of reloadings and accuracy will not be impaired.

Related Article:
Two-Step Sizing and Concentricity


Update - July 2, 2010

People frequently as "How much neck tension should I use?"  I think the following reponse from Paul the moderator at http://www.accurateshooter.com/ is about as good a response as I've seen on that topic.

There is no "best" -- some guns/cartridges work well with .001 neck tension, some others need more... maybe a lot more.



But here's a suggestion that may help put things in perspective. Think in terms of overall 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 (carbon can act as a lubricant; ultrasonic cleaning makes necks "grabby")


4. The springiness of the brass (which is related to degree of work-hardening; # of firings; time between annealings)


5. Time during which the loaded round has sat prior to firing


--and there are others...


You can do this simple experiment. Seat a boattail 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 nominal tension). You'll find the deeper-seated bullet is gripped much harder.


I have also found that thinner necks, particularly the very thin necks used by short-range benchresters, 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 .001. You may find that the bullet in the thin necks actually pulls out easier, though it supposedly has more "neck tension" based on bushing size.


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 inaccurate.


We don't have any easy way to measure "true" neck tension on a bullet.


My point here is that it is overly simplistic to ask: should I load with .001" tension or .003". In reality, an .001" reduction on a thick neck might provide as much or MORE "grip" on a long bullet than an .003" reduction on a very thin-walled case on a short shank.


What I think this means... and this is only a theory... is that I suspect the guys using .001" "tension" on no-turn brass may be a lot closer to the guys using .003" "tension" on turned necks than either group may realize.


This doesn't really provide any answers. You have to go out and test empirically to see what works, in YOUR rifle, with YOUR bullets and powder. 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.


All I'm saying is that the nominal bushing size is not really a satisfactory indicator of the true amount of neck grip on a bullet, or the force required for release. TRUE GRIP () is a much more complicated phenomenon, one that is affected by numerous factors, some of which are very hard to quantify.
 

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