Basics: Resizing - Case Dimension Changes

Resizing - Case Dimension Changes
by: Germán A. Salazar


When you get home from the range with a box full of fired brass, it's time to get to work.  The most fundamental decision to be made is how to resize the brass - will it be full length sized or neck sized?  Some people believe there is a third alternative: partial full length sizing.  Once you've performed the procedures shown in this article, I think you'll understand and agree that neck-sizing is not suitable for Highpower shooting, that full-length sizing is the best way to resize cases and that the third option is no option at all.  I also hope you will have a better understanding of what goes on inside the resizing die.

Some of the noticeable effects of improper resizing are:
  • hard bolt closing,
  • hard bolt opening after firing rounds that have normal pressure, 
  • a click at the top of the bolt opening stroke with normal pressure loads,
  • early case head separation,
  • damage to the bolt locking lugs,
  • frustration leading to assorted venial sins.
Any one or more of these symptoms may present themselves if a case is improperly sized.  You can oversize a case as easily as undersize it and we'll cover all of the possibilities.  A quick refresher on measuring the case might be useful before we begin, so if you're new to reloading, click here to read an article on basic case measurements.

Let's take a look at what the resizing die does and why it does it.  When you take a shiny new factory loaded cartridge out of the box, you expect it to fit into the chamber of your rifle regardless of whether the rifle and cartridge were both made last week at Remington or if they were made 50 years and two continents apart.  If the cartridge is a .30-06 and the rifle is chambered for .30-06, you have the right to expect them to be dimensionally compatible and amazingly enough, they are.  That compatibility is a result of the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) having promulgated standardized dimensions for both chambers and cartridges to ensure compatibility.  The cartridge obviously has to be smaller than the chamber and SAAMI standards call for the largest acceptable cartridge to be smaller in all dimensions than the smallest acceptable chamber.

Case and Chamber Compatibility
Below are the standard drawings for the .30-06 cartridge and chamber.  Compare the dimensions and tolerances of a few key points and you'll begin to develop a feel for the clearances specified.





Once you fire that cartridge, however, all of those dimensions change - 55,000 psi has a way of doing that.  The cartridge case expands in every radial dimension on firing until it contacts the chamber walls.  The case also expands longitudinally to bring the case shoulder to bear against the chamber shoulder.  As pressure decreases, the case springs back from its maximum diameter, although not quite back to the unfired dimensions; it does not spring back longitudinally.  That springiness is what allows you to extract the case from the chamber without having to hammer the bolt open.  The brass cartridges case's springiness, while useful, is limited, that's why we can't simply neck size brass forever without running into problems.

If a cartridge is loaded to an excessive pressure level, the rifle will not work as designed.  Assuming the pressure is excessive but not so excessive as to blow out the primer or cause other catastrophic failure of the case or rifle, you will notice that the bolt is hard to open, especially hard to move near the top of the stroke and hard to retract.  What happened is fairly simple: the excessive pressure caused the chamber walls to expand to a degree that allowed the brass to expand beyond its elastic limit, then as the chamber sprung back to normal size, the brass was firmly captured against the chamber walls with no clearance.  Additionally, the locking lugs may have flexed a bit, allowing the longitudinal expansion to be more than normal and also creating an interference condition when the steel sprung back as pressure dropped.  The case is now firmly stuck.

A case that was reloaded to normal pressure level can exhibit many of the same symptoms of an over-pressure case if it was not properly resized.  Just as the over-pressure case creates an interference fit in the chamber, the improperly resized case does not have enough room to expand as the pressure builds and will cause the chamber itself to spring outward slightly, then trap the case as it comes back to normal.  The cartridge case needs room to expand in order to then spring back enough to extract normally. 

Key Dimensions
The four key dimensions that must be resized properly in order to ensure proper chambering and extraction are:
    Base to cone (colloquially headspace) which SAAMI standardizes at 2.0526" -0.0070" for the .30-06.  In other words, there is a 0.007" tolerance towards shorter, no tolerance for longer.  This, of course, refers to new cases.  Headspace will be checked with a gauge from MCS (203) 775-1013.  The numbers shown in the charts below for the headspace dimension are read right from the gauge.  The gauge is calibrated so that a reading of 0 is equal to the SAAMI minimum length for cone to base on the specified cartridge.  In this case, my readings of about 4 indicate that my chamber is 0.004" longer than minimum, or about halfway into the allowable tolerance of 0.010" for that dimension on the SAAMI chamber print.  That's a comfortable place to be as it ensures that the fired case can go into the die far enough to be resized properly.

    Base diameter is standardized by  SAAMI at a point 0.200" from the base.  This is in the solid web area and will not usually change much.  We will examine a point 0.390" from the base; that is closer to the point at which the .30-06 case has maximum expansion.  Not coincidentally, it is where case head separations typically occur on a .30-06 case.  Base measurements will be taken with a blade micrometer. When you put a .30-06 case into a Forster .308 case gauge, the base sticks out just the amount we need for our base measurement; we'll use that procedure to ensure consistent placement of the micrometer.
    Shoulder diameter, standardized by SAAMI at 0.4410" for new .30-06 cases.  Measuring the shoulder diameter can be a bit tricky, but with care, I think we can hit the same spot consistently. This diameter will be checked with the calipers for convenience as it is a slightly less critical dimension than the base diameter and getting a micrometer on the same spot at the shoulder is very difficult. The calipers, although less precise, are easier to get to the same spot and operate with a single hand while the other hand holds the case.


    Case length, standardized by SAAMI at 2.494" -0.020"for the .30-06.

    Each of these case dimensions must be sufficiently smaller than the corresponding chamber dimension to ensure proper functioning.  In most cases, a reduction of 0.002" from fired dimensions to resized dimensions is sufficient for base and shoulder diameter as well as headspace; sometimes less will do, this depends a bit on the pressure at which the cartridge is fired.  Case length must be monitored as it will always grow and can cause serious pressure problems if it exceeds the maximum allowable length.  While neck diameter is also an essential element of case resizing, we're not going to examine that because all methods of case resizing will adequately resize the neck.

    The next illustration is a print of the chamber reamer used to cut my rifle's chamber.  You can see that the shoulder dimension is specified at 0.340" and the base at 0.474".  Given the specified body taper of 0.16237" per 20 inches, the chamber dimension at the point we measured (0.390" from the base) should be 0.471".



    Let's take a look at a set of fifteen cases in some detail.  We will examine the key dimensions before and after neck sizing, full-length sizing and "partial full-length sizing" using five cases for each method.  The test cases are Lake City 62 Match that have been fired four times.
    Neck Sizing  The first five cases were neck-sized only, using a Redding Competition Series neck-sizing die.  Each of the four key dimensions was measured on each case in its as-fired condition (AF) and after being neck-sized (NS).  The average change in each dimension was then calculated and is shown at the bottom of the chart. 

    Although we expected to see no change in the base and shoulder diameters, you will see that there was, in fact, a small but consistent reduction in those dimensions.  The Redding die uses a sliding sleeve to hold the case in alignment with the neck bushing and apparently, that sleeve is very close in size to the chamber of my rifle and was doing a little bit of sizing.  More importantly, we see that there was no change in te headspace dimension and a slight bit of growth which came from reducing the neck diameter.  Every significant reduction in diameter will always be accompanied by an increase in length. 

    A neck-sized case will usually be a snug fit in the chamber because only the neck dimension has been materially altered and the clearances at the base, shoulder and headspace are minimal.  Upon firing, you will often find that the bolt opens harder and with a click at the top of the opening stroke.  That is because those minimal clearances were insufficient to allow the brass to expand and spring back properly and it is now bearing firmly against the chamber walls.  If the base isn't sized enough, you'll get the click as the bolt begins to cam back for primary extraction. 

    Brass that was neck-sized in this die gave a reasonably normal feel on closing but exhibited all of the tight-fit symptoms after firing.  Closing was normal due to the small amount of sizing being inadvertently done by the sliding sleeve and the gentle shoulder angle of the .30-06.  Most neck-sizing dies don't touch the body at all and even the sliding sleeve type might not contact the case in most situations; this just happened as a coincidence of my chamber and the sleeve dimensions.

    Partial Full-Length Sizing If you've been around reloading for any length of time, you've probably heard someone refer to "partial full-length sizing."  The general idea is that by slightly raising a full-length die in the press, you can use it as a neck die and avoid the other dimensional changes that accompany full-length sizing.  The proponents of this practice usually claim longer brass life as a key benefit.  This is flatly wrong, there is no fudging this point - partial full-length sizing is detrimental to your rifle!  Let's have a look at what happens to the case when a full-length die is used in a manner other than that for which it was designed.

    We took five more cases and measured as-fired (AS) and after the partial full-length sizing process.  In this case, the die was raised 0.020" from its normal position.  As you can see from the chart, the base and shoulder were reduced in diameter just as much as when the full-length die was used normally (see below).  Most importantly, the headspace dimension increased.  This is not a trivial or insignificant matter; an increase in the headspace dimension will create rifle problems.  Because the case does not spring back longitudinally when fired, the headspace dimension of the fired case is essentially equal to that of the chamber.  When you close the bolt on a case with a longer headspace dimension, you are using the chamber as a resizing die and the bolt as your press.  Not only are they not designed for such use, but the stress of closing against an interference fit will quickly wipe the bolt's locking lugs clean of any lubricant.  Pretty soon you'll start to feel the bolt close harder and harder.  By the time you stop to see what happened, there may be significant galling of the locking lugs.  Now you have a big gunsmithing bill ahead; the lugs and lug seats will need to be recut, the barrel set back and perhaps the bolt re-timed.  And guess what?  Since the brass went through all of the full-length sizing dimensional changes, what did you gain?  Nothing, not one darn thing.  You lost quite a bit, however, on the rifle repair and I guarantee that the hard bolt closing didn't help your score one bit either!

    Full Length Sizing  Now let's examine full-length sizing.  If you're a Highpower shooter, this is really the best option; for other forms of shooting it probably is also, but I don't like to comment on disciplines in which I don't participate.  As you'll see, when the full-length die is properly adjusted, you'll reduce the base, reduce the shoulder, reduce the headspace by 0.001" to 0.002", reduce the neck diameter (not shown) and the length will increase and will need to be trimmed.  This is the real life cycle of brass and eventually, after a fair number of resizing operations, the brass will fail, either through case head separation, neck splits, or other forms of failure.  Depending on the exact form of failure and the number of cycles the case lived through, you may need to examine your practices  Ten firings from a bolt action is not unreasonable; more than that is a nice bonus, less is cause for investigation.

    Moving Metal
    The cartridge case is a tapered cylinder right up to the shoulder.  When you squeeze that cylinder down in a resizing die, it will flow forward; both the base to cone (headspace) and the overall length will grow.  When the case shoulder contacts the die's shoulder, it will be pushed back to the proper dimension.  But with either neck sizing or partial full-length sizing, the shoulder never makes contact and we're left with an excessively long headspace dimension.

    Proper adjustment of the full-length die will keep all dimensions within the tolerances and clearances needed for proper operation of the rifle - including maximum accuracy.  Accuracy comes from consistency, and only a full-length sized case is consistently like its mates.  Neck sized cases will  over the course of two or three firings; not only becoming quite tight in the chamber, but will do so to varying degrees, leading to inconsistent bolt operation and inconsistent barrel harmonics.  A fired case hits the inside of the chamber hard enough to initiate an element of the barrel's vibration pattern; do that inconsistently and your accuracy will suffer.  Partial full-length sizing is bad in every aspect and the only remaining thing I have to say about it is that had I used a more aggressive die, such as the Hornady, the headspace growth would have been significantly greater. 

    Highpower shooters need reliable accuracy; it's easy to lose sight of that when someone offers us a new reloading "trick".  I've seen way too many good, even great, shooters withdraw from a match due to problems created at the reloading bench.  Inadequate resizing is a frequent contributor to these problems, which include, among other things: galled lugs, bullets stuck in the rifling, blown out primers, split cases, excessive pressure and more.  Make sure that you don't beat yourself out of a good score before you even arrive at the range - reload carefully and conservatively and learn why things work a certain way, the rewards will be found right on your score card.

    Thanks to Jason Christ for the good looking charts!

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