The technique of estimating pressure was publicized mostly by Handloader magazine in the 1970s and 1980s, I think. The two authors I recall most heavily promoting the technique were Ken Waters (of "Pet Loads" fame) and Bob Hagel. Ken Howell may have done his share also.
Checking some back issues of Handloader, I found an article by Hagel that seems to show most clearly the
technique.
For rimless and rebated rim cases like the 450B, the technique requires a good micrometer, as Hoot notes.
Below are a couple of images with captions, scanned from "How to Detect and Control Pressure", an article by Bob Hagel in
Handloader Number 91, May-June 1981, starting on page 18. The images are on page 21.
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- From Handloader #91, 1981
- press1.jpg (46.5 KiB) Viewed 18194 times
One image shows the measurement of the "pressure ring", which is the bulge that forms on most cases near the front end of the case "web". As the caption notes, this technique
is not particularly reliable.
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- From Handloader #91, 1981
- press2.jpg (51.17 KiB) Viewed 18194 times
The other image shows a measurement over the solid part of the web, which is more reliable.
A couple of notes on the technique as outlined by Hagel and Waters: Before firing, one must mark the points at which the neasurements are to be made. More than one diameter should be marked, because cases do not expand uniformly around their circumference. At least three measurements should be made at each point, and these averaged, and then the two or three point averages should be averaged. (With the 450B's rebated rim, there is no problem with the rim interfering with the measurements as noted in the caption.)
The technique is not especially reliable; Tim is right on with his comments. Waters was very careful to limit its use to comparisons involving expansion of a factory load as a presumed maximum. He made the comparison with his selected load using a case from the same box as the factory load with the primer, powder, and bullet removed, and with his load's primer, powder, and bullet substituted.
Whether the technique is useful or useless is a topic for another thread.
--Bob