Anyone interested in the Pressure Trace instrument should read this
article:https://www.shootingsoftware.com/ftp/Instrumenting%20your%20rifle.pdf. It will get your feet wet on what is and isn't possible to determine about pressures.
Jack,
First, housekeeping (and I am hoping this solves other people's problems in this area):
HOW TO MAKE A WORKING LINK (Windows)
Go to a page you want to link to in your browser and select and copy the content of the address bar at the top. Alternately, go to a working link, as to a YouTube video link, and right-click on it and from the flyout menu select to copy the link location. Back in the composition window you are trying to put the link into, you can paste the link text in and then select it and then click on the globe and infinity icon at the top of the composition window. This the place to place your link. It has "http://" already there for someone typing a link in manually. Delete that or select it so it will be overwritten, then paste your original link, still in your clipboard, into that and then click on "OK". DO NOT paste the text into your composition window and later copy it from your composition window, as the system sometimes truncates that text version. Also, you can skip the first step of pasting the link as text if you instead want to have other text in your composition contain the link. For example, you could write "You can see what I am describing on this video" and then select the words "this video" and then click on the link icon and paste in your URL. This will make the words "this video" turn into blue highlighting and clicking on them will then take you to the video.
On 44-40 pressures, SAAMI barrels have minimum chambers and nominal bore dimensions. To mimic one, you have to get a barrel blank made to their bore cross-sectional area dimensions and then chamber it with a special reamer ground to produce the SAAMI minimum chamber diameters and not run it in past half a thousandth beyond what produces SAAMI minimum headspace. That's ±¼ of a thousandth, which can take some skill to do. Standard reamers are commonly made a little wider so they can be resharpened without losing dimensional precision. I don't know if you followed these practices with your test barrel or not, but as Denton Bramwell points out in the article I linked to, most chambers made with standard reamers can expect to produce lower numbers.
SAAMI test barrels are fired with the cartridge manipulated to ensure the powder has fallen back over the flash hole. They are not tapped against anything to do this, as that packs the powder which can lower its burn rate a little. They want worst case highest pressure, so they just turn the case around to let the powder fall back to the rear, then load it carefully so as to avoid shifting the powder position. If you don't do that, you will get lower numbers than they do.
For rifle cartridges, SAAMI expects no more than 4% standard deviation in pressure. If your readings produce a higher SD than that, you are probably using a sample too small to be relied on to produce a good SD number. SAAMI uses ten rounds. Our own board member, Statshooter, who is a professor of statistics, says 30 rounds are needed. Denton Bramwell feels 15 rounds give him what he needs to know. So, why does SAAMI just use 10? They accept the SD results produced by 10 can be off on the high side by two standard errors (about another 2.5%), so they have a second number in addition to the MAP called the Maximum Probable Lot Mean, or MPLM, that allows the next sample of 10 from the same lot could produce a standard deviation that was different by that much. So, while the MAP is 11,000 psi, they allow they might get an average as high as 11,300 psi from another random sample of 10 from the same lot, and that is acceptable. They have a still higher number, called the Maximum Probable Sample Mean (MPSM) that allows that bullet bonding with the case and other factors can produce an increase in pressure as a lot of ammunition ages, and for the 44-40 that number is 11,700 psi. So, in effect, they are saying 11,700 psi is the safe limit, but, wait, there's more: All those numbers, the MAP, the MPLM, and the MPSM are averages produced by 10 round samples. The individual rounds making up that average will go above and below that average. SAAMI controls that by allowing what they call the Maximum Extreme Variation (MEV) which is the maximum spread between pressure readings for any of the 10 rounds in any sample taken at any time in the life of the lot. For the 44-40, the MEV is 2300 psi (this has to be calculated per the standard's instructions; it is not in a table). Theoretically, you could have aged loads centered on 11,700 psi whose 10 shots had one round as high as 13770 psi that would still be acceptable to SAAMI (though it is extremely improbable as it requires all 9 other shots to be exactly 11,470 psi). Mostly, though, you don't see individual rounds more than about ten percent above the MAP.
Even with all of that care, the fact is the absolute pressures are not exactly knowable. SAAMI reference cartridges are made in lots that are sent around to member's test facilities every two years to watch for reading changes. The members use SAAMI standard calibration methods (see the SAAMI standard) and report their results for both pressure and velocity from SAAMI standard test barrels. Interestingly, these vary more than the SAAMI standard deviation limit. The old 1992 standard gave an example for each method firing the same lots of reference loads, and the copper crushers, using targe tables for calibration, produced over 23% difference in average pressure for 10 shots among 9 labs. The transducers, using hydraulic pressure for calibration, produced over 11% difference in 10-shot averages among 7 labs. The reported results for each set of tests from each lab were, in turn, averaged and the final average was declared to be the pressure value that reference lot produced. This way, in effect, the reference lot pressure in the example was evaluated by looking at 90 and 70 total sample shots for the crusher and transducer, respectively. It compensates for average measurement calibration and operation errors, combined, to give a practical expected correct reading.
That approach may seem a bit tenuous, but understand the objective is to get everyone producing ammunition to the same standard, even if its absolute accuracy is not perfect. When a manufacturer wants to load a million rounds in a particular chambering, he buys a sample of a current lot of reference ammunition for that chambering and fires it in his own test equipment and compares his results to its agreed-upon rated pressure. This gives him a calibration factor for correcting his equipment's output to match the average output produced by the rest of the test labs that measured the lot. In the end, he doesn't load to the raw pressure readings his equipment provides, but to pressure readings corrected by what the reference load told him was his equipment's error factor.
So, if you wanted to compare your strain gauge results to commercial equipment, you would ideally get some SAAMI reference ammunition to calibrate to. Unfortunately, reference ammunition is only sold to ammunition makers or others who can show a real need to have it. Otherwise, I think, they fear a run on it would occur that could interfere with the industry.
One thing you could do is ask Buffalo Bore if they use the SAAMI reference ammunition and, assuming they do, you could probably ask them what their calibrated reading was for their most recent lot. You would still have to condition it in the same temperature it was conditioned and tested in by Buffalo Bore and handle the cartridges to keep powder to the rear, but you could get a correction factor to multiply your strain gauge factor by through shooting it and claim to match their pressure fairly closely. That sort of secondary standard approach is probably as close to absolute as amateur equipment can be made to be.
I notice all Hodgdon's psi-rated loads for the 44-40 exceed the SAAMI MAP, while their CUP-rated loads (in their pistol section) do not. Makes me wonder if someone confused the digits? It's still below the CIP's pressures, but those are measured on a different style of transducer (a channel transducer).
Regarding CUP ratings in manuals, while the correlation between CUP and psi is too poor for reliably converting one unit to the other over a range of chamberings, within a single chambering the conversion by the ratio of the CUP and psi maps within the SAAMI system is going to be close enough for practical work. That is, 11000 psi divided by 13000 CUP is 0.846 psi/CUP for the 44-40, so you can take the CUP numbers in the Lyman Manual and multiply them by 0.846 to get a reasonable expectation of psi. Conversely, dividing psi by that same number will come close to CUP.
