EVERY time we fire a shot, a cartridge case's walls expand outwards and stretch lengthwise, while its base presses against the bolt or breech block face.
These changes stop hot, high-pressure gas escaping to the rear, damaging the firearm and injuring us. Then the case contracts a little, allowing us to extract it.
In the September 9 column in The Weekly Times, we saw why handloaders (shooters who assemble their own live rounds) must know when gas pressure starts to push the cartridge case brass past its elastic limit.
Then we discussed indicators of excessive pressure, namely extraction difficulties and brass flow in the case head.
Though the brass flow indicators are important, their innate subjectivity means some people miss the early warning signs. So let's look at an objective method: measuring expansion of case heads. Yes, you've got it for safety's sake, avoid getting a swelled head.
To ensure we're all using the same language, key terms are illustrated in our picture (right).
Before the initial firing, each case head is measured at pre-determined points, right next to the front edge of the extractor groove but on opposite sides.
The heads are re-measured after the second and later firings, using exactly the same points. This tells us how much they've expanded, compared with their initial diameter.
The vernier calipers owned by most handloaders cannot cope with the task as they do not offer enough precision. Instead, we need a micrometer able to read in ten-thousandths of an inch or thousandths of a millimetre, the latter being interpolated if necessary.
Standard micrometers may not reach the measuring points after the initial firing, thanks to the expansion ring that develops in the case walls next to the web.
The solution is a micrometer with a blade-shaped anvil and spindle, or else a good tubing micrometer.
A little head expansion is normal in many rounds, so how much is too much? We can use these three guidelines:
Maximum head expansion for mild or moderate cartridges with relatively lightly constructed cases, for example the .22 Hornet and the .30-30: .000-.01mm (.0003-.0004in) more than virgin diameter.
Maximum for the more powerful cartridges whose cases are more robustly constructed, for example from the .14/221 Walker and .17 Mach IV through to the .30-06 and other big rimless designs: .013-.015mm (.0005-.0006in) more than virgin diameter.
Maximum for the belted magnums: .015-.018mm (.0006-.0007in) more than virgin diameter.
Now for two important provisos. First, we can't take a "before" measurement from one case and compare it with an "after" measurement from another case.
We must keep track of changes in individual cases from go to whoa.
Second, strain hardening of the brass occurs with repeated firings and proceeds at slightly different rates in individual cases. Result: the more each case is fired, the less reliable the guidelines' figures become.
