What eddybo said.
"Super glue" the stain gage to the chamber. Get very accurate chamber measurements then go to shooting.
I figure there are plenty of variables with this or any other method thus I worked up the load for that rifle and called it standard for pressure.
I then compared everything else to that pressure curve. I got really anal about it but discovered that pressure curve maximum values and the time of the "peak" pressure with respect to bullet travel varied enough for me to give up on the idea as being too techie.
Pressure curves showed more variation than I would have expected even when ES was less than 10.
Having said that, the evidence of secondary pressure spikes with the 338 RUM still has me confused. The fella @ RSI said they were'nt unusual??????
I"m going to fire the thing up again when I make another attempt at forward ignition. Forward ignition has been the only thing I've messed around with that lowered recoil, decreased ES and consistently improved accuracy with out having to try different powders to achieve the same results.
However, I did launch some brass tubes down range which really confused the chrono.