Measuring the Case
by: Germán A. Salazar
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.
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!
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!
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!
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!