Q: I am puzzled regarding the proper method and criteria to use when measuring case-head expansion to determine relative and maximum pressures for a particular rifle. This is after I consulted three different reloading manuals and got three different methods.
The first manual recommended measuring case diameter just ahead of the case web and that the expansion from fired factory rounds should not be exceeded; the second manual suggested taking the measurement just in front of the extractor groove and stated that an increase of .013mm (.0005”) be considered as an indication of high pressure; the third manual said that measurements should be taken from only once-fired cases, and cases should be used for only three reloads due to the brass hardening. Their safe pressure criteria was an average case head expansion of .008mm (.0003”) with no readings over .0005”.
One authority, a gunwriter, says that measuring the case diameter at the pressure ring and then comparing it with the measurement taken from factory rounds is the most accurate method.
I find all this contrary information to be contradictory and confusing, particularly the criteria of how to go about determining a relative indication of maximum pressure levels. Could you please tell me the best method to use — the one that gives the most accurate readings?
Donald Marshall
A: Firstly, let me state that the methods of measuring case head expansion described in the manuals you consulted are not contradictory. There are two different places at which to measure the diameter of fired cases — over the solid brass base in front of the extractor groove or over the pressure expansion ring.
Both systems work, but because the solid base is least subject to expansion, readings taken there will be extremely small due to the minuscule increments, and the fact that pressures in the lower ranges, say less than 30,000psi, may not register.
Measuring case diameters at the pressure ring, on the other hand, will readily show differences as low as 15,000psi and readings of variations are likely to run from .13mm to .04mm (.005” to .0015”) — much easier to measure with a normal micrometer. This is why I prefer to measure over the pressure ring.
Another advantage: full-length resizing usually irons out a pressure-ring bulge so that cases can again be measured after the next firing, whereas the shell-holder head prevents a sizing die from contacting the solid base section.
One point I would like to draw to your attention: some factory cartridges, especially the magnums, are loaded to maximum pressures and it would be safe to consider their expansion to be maximum. Older cartridges like the .30-06, for example, aren’t loaded quite as hot, and in a strong bolt-action rifle may accept a .13mm (.005”) increase in expansion, while others like the 7×57 and 8×57 may withstand an increase of from .03mm to .04mm (.001” to .0015”) case expansion.
The truth is, there’s no set-in-stone criteria for permissible case expansion, a point I like to make clear about each calibre you may reload for.
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