Getting Consistent Ideal Headspace
When Full-Length Resizing M.L. McPherson
Synopsis: For bottlenecked-case reloads used in a typical chamber, conventional full-length case resizing generates about 6/1000-inch functional headspace between case and chamber. When a shooter chambers such a round, the momentum transferred by striker impact and the energy generated by primer ignition will drive the case as far forward in the chamber as placement of case and chamber shoulders will allow. Then, as pressure increases, the case body will expand and bond with the chamber wall. Finally, with normal loads, chamber pressure will drive the
case head back until the bolt supports it. This will significantly stretch, and thin, the case wall near the body-to-web tran- sition. Subsequent full-length case resiz- ing will squeeze the body (which forces the shoulder even farther forward), then drive the shoulder backward, thereby significantly lengthening the case neck. Eventually, it is necessary to trim such cases. Handloaders are familiar with this process. The more excess functional headspace the chambered case has, the more case stretching will occur with each firing and the more neck length- ening will occur with each full-length resizing. Conversely, the less functional headspace the chambered case has, the less case stretching will occur. The ideal goal for full-length resizing is to drive the case shoulder back only enough to allow free chambering. The question is, how to best do this? HISTORICAL METHOD (WHAT I READ AS A NEW HANDLOADER) In a bottlenecked case, conven- tional full-length resizing leaves the case-body length significantly shorter than the chamber-body length – this dif- ference represents functional headspace. When we fire such cases with full-power loads, case stretching is inevitable. The striker impact and primer explosion will drive the case fully forward in the chamber, chamber pressure then will bond the case body to the chamber wall, and then chamber pressure will drive the case head rearward until the bolt supports it, thereby stretching the case body. In loads with typical functional headspace, this stretching exceeds the
elastic limit of the case and therefore engenders permanent thinning of the case body forward of the web; eventu- ally, such cases are unsafe to use. After that, progressive case-wall thinning eventually will lead to a case separation, which can destroy the gun and possibly injure or kill the shooter and bystanders. (Perhaps you have experienced
or seen harmless case-head separations – most serious shooters have. This can easily lead one to suspect that separa- tions are harmless, which is certainly far from accurate – many guns have been destroyed and many shooters have been hurt when such a separation occurred while chamber pressure was near the peak and when the case head also failed to seal the action. I have expe- rienced this; fortunately, I was shooting a Savage which features excellent gas control measures and therefore, other than blowing off my shooting glasses, no harm occurred.)
As variations in the resizing pro- cess result in progressive increases in headspace the resulting case stretching during firing progressively increases and case life progressively decreases. Therefore, a fine goal is to minimize headspace and thereby minimize case stretching, which is apt to improve ac- curacy while reducing (and sometimes eliminating) the need for case trimming. Historically, the functional head- space reduction method, as recom- mended by many pundits and sug- gested in many loading manuals, was to adjust the full-length sizing die so that the shellholder could not drive the
Sketch of a cross-sectional view of a chambered belted-magnum round in a typical chamber: This clearly shows excessive clearance between the case shoulder and the chamber shoulder. The case belt will hold the case rearward and therefore will determine functional headspace. However, working of the case shoulder during firing and reloading will be excessive and accuracy and case life will suffer. It generally is possible to improve accuracy by adjusting the resizing operation to move headspace control to the shoulder. This usually improves accuracy and always improves case life. In a conventional bottlenecked rimless case, the striker impact and primer explosion will drive the case forward until the chamber fully supports the case shoulder. This will reduce accuracy, increase case stretching, and reduce case life.
Page 134 Winter 2013
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