Have been in plenty of pressure labs, and none of the techs at the best place that much faith in results from strain gauges. They say strain gauges are useful for certain purposes, such as comparing average pressures from factory ammo and handloads in the same barrel, but not for the sort of statistical analysis indicated. It's sort of like the difference between a Shooting Chrony and a lab-level Oehler. I am sure a bunch of people will object to this statement, but that is what the boys who actually work with piezo-electronic equipment have told me.
You have it partly right, but may have misunderstood why, unless I'm not following what you meant. There's nothing wrong with the accuracy of strain gages - for measuring strain. They are very accurate when done right.
The disconnect is in correlating strain measurements to pressures. You can strain gage a barrel and record data when you fire it, but you aren't measuring pressure, you're measuring strain. Drawing a correlation between strain and pressure on that particular barrel is an estimate at best, because you don't know what the pressure really is. That's why this method can be useful for comparing a known load to something new, but not so good for absolute pressure measurements.
Disclaimer - I don't work for an ammo or powder company, but am one of the "boys" that work with strain gages and other instrumentation on a regular basis.
