Basics: Measuring the Case

Measuring the Case
by: Germán A. Salazar

This is another introductory reloading article, there will be little here for the experienced reloader but those who are new to reloading might find something useful in these words and pictures.  All of the pictures can be enlarged by clicking on them.


Introduction
The cartridge case is a seemingly simple vessel which holds the primer, powder charge and bullet, but in reality it is a very dynamic part.  Not only must the case hold the other components, it must enter the chamber easily, release the bullet on command and then exit the chamber without undue difficulty.  In order to accomplish these functions, the case is actually a complex little item which varies in thickness, hardness and diameter at every point along its length and we must be aware of these variables.  Additionally, there are a few linear and diametrical dimensions which we must be able to measure and control to ensure the safe and reliable functioning of the rifle.

Before we measure anything it is important to have an understanding of its characteristics and of the required resolution in our measurements as these factors will determine how we measure and with what instrument.  Some dimensions can be measured directly, others can only be measured indirectly.  We will demonstrate each of these and explain the measuring technique and instrument used.  With respect to diametrical measurements, the most important thing to understand about the cartridge case is that it is tapered.  If it were not tapered, it would be extremely difficult to form, to chamber, to extract and to resize.  The tapered form makes all of these processes relatively simple, but it makes measuring the case a bit more complicated.  Most importantly, it means that when we measure a case at a given point along its length, we need a means to return to that same point or the readings will not be comparable.  The taper and the round shape of the case also influence the type of measuring instrument used.  Some dimensions are linear and simpler to determine, let's begin with those.

Case Length
The first dimension every reloader learns to check is case length.  The reason is fundamental safety - if the case is too long for the chamber, the case mouth will be jammed onto the bullet, greatly and dangerously increasing pressure as the powder charge burns and the bullet does not release as designed.  For each case that is standardized by SAAMI or CIP there is a maximum acceptable case length and a minimum or "trim-to" length.  These dimensions are listed in all reloading manuals and should be followed without exception.  Using the .30-06 as an example, the maximum case length as listed in the Sierra reloading manual (which we will use for all references) is 2.494".  I trim to 2.475" as shown, which gives me about three resizings before it's time to trim again.

Case length can be measured with an ordinary set of calipers, whether of the vernier, dial or electronic type.   A measurement resolution of 0.001" which is within the capability of even the most modestly priced calipers is sufficiently close for this type of measurement; there is no need for more precise measuring instruments such as a micrometer for this application.  If your electronic calipers show a fourth digit after the decimal point (ten-thousandths) ignore it for this purpose.  There is also no need to try to make each case absolutely identical in length; if you get them all to within 0.002" of each other, you're doing fine.  Tooling and technique will influence the consistency of your trimming, but with practice and good measuring technique, you'll have safe cases.

Because resizing causes the case to grow in length, case length is measured after resizing.  However, you may want to measure before and after sizing to see just how much the case is growing.  In those situations, be sure to remove the primer with a decapping tool or die (not the sizing die) before measuring the case as it can influence the reading if it protrudes at all from the case head where the caliper jaws rest. While you're examining the case, make sure there are no burrs on the edge of the rim as can be caused by the extractor (particularly with semi-autos) or from being dropped on concrete surfaces.  If so, the burr can be carefully filed off or the case discarded depending on the severity of the damage.  Use the wide part of the caliper jaws to get solid contact for the case.

Another way to check length is with a case gauge.  In reality, the gauges, such as those made by Wilson or Forster, are a more useful tool for checking length than calipers.  The gauge checks neck length, which is actually the critical dimension and can't be measured directly because you cannot accurately and consistently place a set of caliper jaws at the base of the neck.  By using a gauge, you can see the minimum and maximum length of the neck as steps on the gauge and by running your finger across the gauge with the case inserted you will know if the case is too long.  After trimming you can quickly recheck the case to ensure that it is short enough but not too short.  This is the only instance where your finger can actually give a better result than a measuring instrument! 

Neck Diameter
There are several points in the loading process at which knowing the neck diameter is important.  First, the neck diameter of the loaded cartridge must be sufficiently smaller than the chamber neck diameter to allow the case neck to expand and release the bullet.  Second, the resized case neck must be small enough to provide an interference fit for the bullet when seated so that the bullet doesn't move through handling and during the chambering process.  The case neck is slightly tapered, although not as dramatically as the case body, so while we should be careful to measure at the same place along the neck's length, it is not an ultra-critical point. 

The caliper is a useful instrument for measuring case neck diameter; its resolution level being sufficient for this purpose.  The main caution is to avoid the very end of the neck (the case mouth) as it can have slight irregularities.  The picture shows a good location, just a bit back from the case mouth but still towards the front of the neck.  Use the narrow part of the caliper jaws for this measurement to avoid undue influence from the slight taper in the case neck.

Unless you have the reamer print for the reamer used to cut your rifle's chamber you probably don't know the exact diameter of the chamber neck.  The chamber neck diameter can be indirectly measured by measuring the neck diameter of a fired piece of brass and adding 0.001" to that dimension.  This technique is accurate enough when using relatively new brass (three firings or less).  If this method shows that you have at least 0.004" neck clearance in the chamber (calculated chamber neck diameter minus loaded round neck diameter) then the brass is safe to use. 

If your result indicates less than 0.004" clearance you should take further steps to really determine the chamber neck diameter; obtaining a reamer print is the most useful way to do this.  Additionally, for clearances below 0.004" you should consider neck turning to ensure that every cartridge has adequate clearance.  Unfortunately, there is enough variance in different lots of brass that when your chamber neck is close to minimum with one lot, it may be dangerous with another, slightly thicker, lot or brand.  As an example, Lapua brass is significantly thicker than Winchester brass; my .308 has a chamber neck dimension of 0.336" (below SAAMI standards) ammunition loaded in Winchester brass measures 0.333" and is safe (though I neck-turn to that dimension to make sure), whereas ammunition loaded in unturned Lapua brass would measure 0.338" and will not even chamber.  Neck clearance is a critical safety item and must be checked!

Headspace
Headspace, as the term is used here, is a linear measurement from the center of the shoulder to the base of the case.  A moment's thought will show that this dimension cannot be directly determined with calipers, micrometers or any other measuring instrument - it must be gauged.  The accuracy of a case gauge is dependent on the case shoulder making contact with the shoulder inside the gauge.  The contact point must be clean and free of lint, dust or any other sor of debris.  Make sure that both the case and the gauge are clean before beginning.

Checking headspace is a routine part of reloading and should not be overlooked; as brass ages through repeated use, you may need to adjust your resizing die to maintain the desired headspace.  The base end of the case gauge has two steps; the case head of a fired case should be somewhere between them if the rifle is properly chambered.  Take a reading with the calipers over the case head and avoiding the high step (which should be higher than the case head), make sure to decap the case before taking this reading.

After resizing, the case head should be no lower than the low step on the gauge, but we want a better measurement than that.  Once again, using the calipers check the case while avoiding the high step.  If your sizing die is properly set up, you should be 0.002" below the fired, unsized case reading. 

In checking with a gauge, one will use calipers over the case and gauge and, again, the resolution offered by calipers is sufficient for this purpose.  Normally, we try to set the shoulder back (reduce headspace) 0.002" in the full-length sizing operation, the caliper can easily measure this. 

The photo shows the use of the caliper on the case gauge.  The measurement must be taken before and after resizing because it is the difference in those two conditions that we are itnerested in determining.  As noted before, the primer must be removed and the base examined for burrs prior to taking these measurements. 

There are other tools for measuring headspace such as the MCS tool (Mo de Fina (203) 775-1013) which is used by inserting the case into the gauge and screwing an indexed cap over the gauge.  The indexing marks are read directly and provide a quick reference for "before and after" headspace.  Another popular method is the mini case gauge made by many gunsmiths when chambering the barrel, this is used just like the full length commercial case gauge, by clamping the case and gauge with the calipers.  For a more detailed discussion of headspace, please see the earlier article about that topic.

Base Diameter
Measuring the base diameter of a case is not part of normal reloading practice.  However, there may be instances when it is useful, either as a method of comparing the relative pressure of two loads. or to determine if insufficient sizing is the cause of hard extraction.  While we won't cover those topics specifically in this article, the technique for measuring the base is within our scope.

Our old friend the caliper isn't precise enough for this job.  We need resolution under 0.001" which is really the caliper's practical limit.  A micrometer is the correct tool for this job, but not just any micrometer.  Remember our earlier discussion regarding the continuous taper of the case?  That not so small detail means that a micrometer with standard round anvils can't be used because the contact area of the round anvil is too large to capture the diameter of a specific point along a tapered case.  The solution is a blade micrometer which has thin, flat anvils and will take a point reading along the taper. 

The next problem to solve is taking a reading at a specific point and being able to repeat that point to check a series of cases meaningfully - or the same case before and after resizing.  In the pictures you can see how I've set up a measuing fixture from a few normal items: a small steel plate, a piece of leather (vise jaw pad) and a small c-clamp.  All of these items stay together in my tool box so that the setup can be repeated over time.

When measuring base diameter, it is the change from one condition to another (unsized vs. sized, or fired with load A vs. fired with load B) that interests us.  The micrometer gives us resolution below 0.001", in fact, depending on the micrometer, you may be a couple of decimal points beyond that - but that doesn't mean you should believe the numbers!  A micrometer, especially a modern digital version has the ability to resolve to an incredibly small degree, however, the accuracy of those numbers is dependent on very skilled and refined technique on the thimble as well as temperature, material and other factors.  For our purposes (ham-handed reloaders measuring soft brass at varying temperatures), any two measurements taken with care and within 0.0003" (three tenths) of each other, should be considered to be the same.  If your micrometer reads below tenths, cover that last digit with tape, it'll just distract you!

Neck Thickness
Measuring neck thickness is often useful, and is essential when turning necks.  If all you need to do is determine chamber neck clearance, measuring loaded diamter and determining chamber neck size as described earlier will suffice.  However, when we get into more precise operations, knowing the exact thickness of the case neck is important.  Here again, we need a specialized micrometer, in this instance a ball micrometer.  The case neck is, of course a round surface and we are interested in a specific point thereon.  Trying to measure neck thickness with calipers is useless, irregularities at the case mouth and slight variations in thickness along the length of the neck will degrade the accuracy of any reading.  Worse yet, the caliper jaws have a certain amount of width, flat surface, which does not conform to the round surface of the case neck; this results in a gap, however minute, between the surface of the jaw and the inside of the case itself and no such reading can be accurate.  The ball anvil on a ball micrometer makes point contact with the interior of the case neck and the flat outer anvil also makes point contact along the opposing point on the exterior of the neck.

As with the case base measurement, it is important to create a method that allows you to consistently measure the same spot on the case neck.  The case neck tapers from mouth to base, so if you aren't measuring the neck at the same place along its length each time, you aren't getting useful results.  You can see in the photos that I've made a small step on the micrometer for the case to rest on.  This allows me to read the same place each time.  If you look carefully at the picture of the leather vise jaw pad used in the case base measurement, you'll see where our little step came from.  Be resourceful!







The Sinclair case neck micrometer, also shown, doesn't have quite as convenient a way to create a case neck rest, but with a little thought and creativity, I'm sure you can come up with a solution.

The earlier article about neck tension has more information about case neck thickness and related topics.
Case Neck Concentricity
Concentricity checking is really more a measure of your reloading die's performance than of the case itself, but the quality of the case will influence concentricity as will some case operations such as neck turning.  There's no great mystery here; set up the tool to read on the case neck about 0.125" in from the mouth, make sure everything is clean and go slowly.  Any operation with a dial indicator is meant to show the reading at various points, not to read in a continuous sweep of the piece.

Case Body Thickness Variance
This is a really specialized operation which is well beyond basic reloading.  I include it here only to make you aware of it's possibility and if you are interested, please read the earlier article on this topic.

Concluding Thoughts
There are obviously a lot of measurements that can be taken on the cartridge case and in some cases, more than one way to take them.  However, the first two that any new reloader must learn are case length and neck clearance, these two are safety concerns and if overlooked can results in serious damage to the rifle and injury to you.
 

All contents Copyright 2012 The Rifleman's Journal