der (such as a 243 Ackley Improved case has) and a thick, hard case shoulder resist shoulder setback to a much greater degree. The harder it is to force the case forward, the more marking we will see with any given amount of case head deformation into the extractor hole. In typical modern bottleneck case
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designs, degree of shoulder toughness varies by a factor of at least 25, depend- ing upon shoulder width and angle, and case shoulder thickness and hardness. In the given example, 358 Winchester versus 243 AI, when using otherwise identical cases, variation in resistance to shoulder setback exceeds five times. Comparing the 35 Whelen to
the 30/110 SMc™ gives an extreme example. The Whelen has a narrow (about 0.026-inch width), gently sloping shoulder (17½ degrees) and the brass of the case shoulder typically is about 0.015-inch thick. The 30/110 SMc has a wide (0.165-inch width), sharp, elliptical shoulder (effectively about 45 degrees), made of brass that is about 0.022-inch thick. If the brass of both case shoulders were equally hard, the 30/110 shoul- der would be about 22 times harder to move than the 35 Whelen shoulder. (In reality, forming the 30/110 shoulder involves far more working of the case and therefore, without annealing, brass in the 30/110 shoulder is far harder, so difference is resistance to shoulder set- back typically is even greater.) As these examples show, even if
two case heads are identically hard and the ejector plunger holes are identical, chamber design and case variations can create a situation where the same pres- sure will show a significant degree of polishing (and unusually difficult bolt lift) on one instance, with almost no polishing (and relatively normal bolt lift) on the other instance. Measuring Casehead (riM) expansion
Now, we get to what might be
the single best method of this type — measuring case rim expansion. Before addressing this, let us consider what happens when we fire a round. As pressure builds inside the case, the primer pocket interior is pressurized, essentially in lock step with chamber pressure. So, chamber pressure is push- ing outward on the primer pocket walls just as hard as it is pushing outward on the case walls.
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The difference is, in the latter in-
stance, the chamber is supporting the case walls. As the case pushes against the chamber, the chamber stretches exactly enough to create exactly enough resistance to prevent further stretching. This is called a normal force and if you are sitting as you read this, you are feeling such a force right now on your butt. The chair is pushing back exactly as hard as your butt is pushing on it. Conversely, along the flash hole length and inside the primer pocket full length of the so-called solid portion of the case head, with any reasonably safe load the case never expands enough to touch the chamber walls and therefore the case head must support the entire applied stress without deforming. As peak chamber pressure gener-
ated by tested loads increases, eventu- ally, the case head, and rim, will expand enough so that those cannot return to original diameter (expansion will exceed the elastic deformation limit of the case head, which then will enter the plastic deformation range). Taking This MeasureMenT In my opinion, this cannot be done
with a sufficient degree of accuracy us- ing less than three identically loaded rounds and with anything less accurate than a quality ten-thousandths-inch micrometer. I always try to average the readings from at least three cases. Now, the question is, how to best
measure this expansion. For an average handloader, the best place to take this measurement is across the case rim. It is important to measure across the same diameter of the rim because variations of a few ten-thousandths-inch matter here and no case rim is perfectly round. Nor- mally, I measure across, from the first letter in the maker’s name abbreviation, and I check to make sure the extractor and ejector have not marred the rim face in critical areas. When using a standard microm-
eter, if we try to measure across the case head, just forward of the rim, what we actually end up measuring is expansion of the hollow case body because the mi- crometer faces are wider than the solid portion of the web is thick. The side of the micrometer face that is forward on the case body extends past the solid web — see drawing, page 86. Belted Magnum cases usually do not create this problem but I have
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