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Old February 6, 2013, 11:32 AM   #10
Join Date: March 4, 2005
Location: Ohio
Posts: 12,133
Thanks for the offer, but I have my wife's repeated assurances that my head is fat enough already.

The plots I combined and curve fit to are in the old U. of M. study done by Dr. Lloyd Brownell in 1965. Harold Vaughn referenced the study in his book, Rifle Accuracy Facts, but suggested that because Brownell relied on the Lame Theorem for hoop stress and strain on an infinite cylinder to calibrate his gages, that his numbers were likely about 20% high. So I knocked the Brownell numbers down 20% for that plot. Subsequently, I've had some other data to suggest Brownell probably wasn't quite as far off as Vaughn suggests, but have not redone the plot, as the difference will simply be proportional. Just allow that the changes it shows may be small by maybe ten percent or a little more. Not enough to change anything in principle, though.

Brownell suggests the explanation for the curve shape is that deeper seating allows more gas to bypass the bullet before it moves far enough forward to obturates the bore, thus putting a brief stall into the build-up of pressure. At the same time, that deeper seating is reducing the powder space. The former dominates to the left of the pressure minimum on the curve, but the latter comes to dominate to the right of the dip.

Two things to note: One is that the bullet used by Brownell was a round nose design, and a spire point is going to cut the flow off over a longer span, assuming his theory is correct. The other is that, unfortunately, Brownell did not test an actual jam into the throat, to check the static friction theory of pressure rise caused by a bullet in the throat. Light contact with a leade cut to different angle than the angle of departure of the bullet ogive off the bearing surface won't see much of that because the contact area is then so small.

Note below a disagreeing plot from RSI's web site (reproduced here with Jim Ristow's permission) showing a larger change in pressure over a 0.030" shift back from land contact. The upper three traces are "in contact with the lands", but how hard the contact is was not mentioned (could be a jam), while the lower four are 0.030" off the lands. In this case it works out to 250 psi difference per thousandth. Unfortunately, there's no way to tell whether being, say 0.005" off the lands would have produced something closer to what Brownell's curve looked like up to that point.

Some further testing is needed. I may get to do some of it this summer in .30-06, but with either 175 grain MatchKings and 150 grain Hornady FMJ's. I have one of the Pressure Trace instruments, but just wish it had more than 8 bits of resolution. The numbers on the plot below should really be rounded to the nearest 200 psi to better represent the resolution limits.

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