the round in the test gun. When I pulled the lanyard, the computer reported in excess of 75,000 psi. My friend went a bit pale — he knew that he should have known better than to make such a pre- diction. Murphy’s law was handy and pleased to remind him! If you can honestly claim that you
have not run into this sort of situation, just wait! Chances are very good that eventually you will no longer be able to make that claim. To repeat, unless we know how
to observe pressure signs, we cannot assume that any particular factory load or handload is safe when fired in any particular gun. So, even if we precisely follow a correct recipe, we are not neces- sarily assured of a safe result. Some cases introduce independent
problems that exacerbate this situation. The prime example is the Hornet. With this case (even disregarding headspace issues), charge variations of only a few- tenths-grain can represent the difference between a load that seems to be perfectly normal and a load that ruins cases by expanding the case head sufficiently so that it will not hold a primer. Beyond that point, anything that results in a load that generates a bit more pressure can create a potentially dangerous loading. So, let us consider basic pressure
indicators, assuming the gun is in good order and has a modern, strong action, which generally we can consider as capable of handling the pressure gener- ated by any normal load. This “sufficient action strength” assumption is critical. If you do not understand all that this implies, please consider the following paragraphs very carefully before con- tinuing with this thesis. Gun Limitations
Concerning chamber pressure
and safety: All guns and ammunition are akin to a chain; all chains have a weakest link. As the strain associated with ever-greater chamber pressure increases, eventually something must fail — some “weakest link” will always break. If that weakest link happens to be the gun, the result can be devastat- ing. If pressure is sufficient to result in a catastrophic case failure, escaping gases can indirectly destroy the gun, which also can be exceedingly dangerous to both shooter and bystanders. Consider what typically happens with a double-charge in almost any re-
Page 88 July — September 2011
volver. When one drops the hammer on such a load, the cylinder typically fails, creating at least two separated pieces that blow off the top strap. (Amaz- ingly, this usually results in no harm to shooter or bystanders, but who wants to tempt fate in that manner! And it is an expensive demonstration.) Con- versely, with a slow increase in applied pressure from one shot to the next, as we would see while developing loads for maximum velocity, and when such loads are used in a much stronger type of gun, we would expect to see other manifestations of excessive pressure long before pressure was sufficient to result in failure of the gun. Definitions:
âStress: force applied — in this
discussion, derived entirely from cham- ber pressure; âStrain: response to stress — in
this discussion, case or action stretch- ing, whether elastic, permanent, or catastrophic; âElastic Strain: reversible defor-
mation; âPlastic Strain: permanent defor-
mation. With strong, modern rifles, cham-
bered to use smaller-diameter cases, usually, we can trust that the action is far stronger than the case. However, this does not mean we can routinely submit the action to unusually hot loads with the certainty of complete immunity because the firing of every such load stresses the action to a greater degree than that for which it was proof tested. This fact, and the mechanism
through which proof-load testing works, leaves us without any way of knowing how many such shots the gun will endure before it fails. Any such failure will always be catastrophic (explosive — disintegrating, rather than simple — deformational). A simple failure, such as a swelled chamber, occurs from firing one wildly overpressure load in a gun that has not been repeatedly abused. A catastrophic failure occurs after using many loads that generate excessive pres- sure and always is of the brittle-failure type — the overstressed portion of the gun explosively disintegrates. Simple failures ruin guns. Cata-
strophic failures ruin one’s day, or worse.
Note that one extreme over-pres- sure load can generate a catastrophic
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