CASE RIM EXPANSION MEASUREMENT - A Better Method
CASE HEAD EXPANSION MEASUREMENT - Problem with Trying to Take Measurement Forward of Rim, when using a Conventional Micrometer
One disadvantage of this system is
that it cannot be used while fi ring shots for accuracy. Another disadvantage is that it requires screen changes after fi r- ing each shot. Using it was an awkward and time-consuming process. It also was expensive; each shot consumed about 10 cents worth of screens, which, at that time, typically doubled the per-shot cost. Needless to say, I was not eager to fi re 20-shot strings to gather data on ve- locity and ballistic uniformity, as I often have done since getting an Oehler 35P. My friend then fi red a few of his
Page 86 July — September 2011
Left: CROSS-SECTION OF PRESSURIZED CHAMBERED CASE - Representation of 22 Hornet case (approximately to scale) when chamber pressure is near maximum. Arrows represent forces acting upon case and primer surfaces; length of arrows refl ects approximate magnitude of force (longer arrow represents greater force). Case walls have some internal strength; hence, normal force from chamber (see text) pushing in on outside of case walls is not quite as great as chamber pressure force pushing out on inside of case walls. Similarly, because case walls support some thrust, normal force of bolt pushing up on head of case is not quite as great as chamber pressure force pushing down on web of case. However, regardless of these complications, unless a catastrophic failure occurs, necessarily, all forces must exactly cancel. A factor that is more important to this discussion is represented by the fact that the chamber is not applying any normal force to the portion of the case exterior between the bolt face and the front end of the case web. Through that region, 100% of the force pushing outward on inside of case is (necessarily) countered by internal case strength and (according to theory), nothing else. This case strength comes from brass hardness and thickness. In response to applied force from chamber pressure, case head elastically swells; as applied force increases, swelling increases; eventually, applied force exceeds case head strength. At this point swelling transitions from elastic (reversible) to plastic (permanent) deformation and case head (rim) diameter begins to permanently increase — the greater the applied force, the greater the permanent increase in diameter. Beyond this point, things get progressively uglier with the ultimate result that the case head liquifi es, which usually results in destruction of the gun, or worse. Bridging strength is an issue. I have attempted to represent that here by showing that force pushing back on case rim diminishes toward perimeter and that force pushing back on case walls diminishes near case web — these forces do not disappear at a specifi c location because the case bridges (or carries) force, through internal strength. This factor (bridging strength) complicates this analysis but makes no difference in the ultimate result — necessarily, all forces must balance. General characteristics of what happens to a centerfi re cartridge case demonstrated in this drawing apply to all cartridge cases, regardless of size, shape, rim type, or other details of specifi c design. Details ignored here include firing pin and associated primer cup indentation. As noted in text, primer cratering (properly, partial blanking) and piercing (properly, blanking) refl ect many characteristics of gun and load and are practically useless as pressure indicators. Comment on details of sketch: Most 22 Hornet chambers have far more headspace (room for case rim to move endwise in the chamber — up and down in this drawing). This sketch represents approximate degree of headspace in my Savage M-40, which is ideal. Proper headspace extends case life, improves accuracy, and improves safety. Many factory 22-Hornet-chambered guns have so much headspace that, upon fi ring, factory loads will partially or fully separate the case head! Primer and striker drive case as far forward as it will go, then chamber pressure locks case walls to chamber and, fi nally, forces case head back until bolt supports it. With excessive headspace, this can stretch and thin case walls permanently. Stretching usually occurs just forward of web. If stretching is suffi cient, it will result in a cracked or separated case, which can lead to a dangerous gas leak. In this sketch, I have represented no thinning because degree of stretching in this rifl e is unlikely to result in visible thinning at this scale. It also is apt to be within elastic range of case deformation and therefore unlikely to result in permanent case-wall thinning.
remaining 80-grain factory loads, for velocity. Then, he fi red fi ve more, for accuracy. This gave us something with which we could compare both accuracy and velocity of our 85-grain handloads. He then fired several of our 85-grain handloads, for velocity. Ve-
locity was similar to the factory 80-grain load. The cases extracted normally, primers looked normal, and the case heads looked normal. Then he fi red fi ve of our 85-grain
handloads, for accuracy. When we walked to the target, we saw that the
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