Check this web site for details. One excerp is copy-paste below.
http://www.shootingsoftware.com/pressure.htm"A strain gage is simply a small piece of polymer material with a fine conductive grid on its surface. As steel around the barrel expands the gage is stretched and current flow through the grid is altered. The PressureTrace module samples these current fluctuations (1 sample per 10 micro seconds/350 per shot), stores it in a memory buffer, then automatically downloads the raw data to the PC via a wireless connection where it is analyzed and plotted. Up to 10 shots can be overlayed and compared on screen at a time."
This seems to be one of the more popular solutions.
