Also probably good to remember, while we're discussing pressure, that the SAAMI service pressure standard is the same for both 2 3/4" and 3" 12ga loads: 11,500 psi. The difference comes in recoil, because of the heavier shot charge that can be stuffed into a 3" hull.
There is no such thing as a "SAAMI service pressure standard." The 11,500 PSI figure given is the SAAMI "Maximum Average Pressure" (MAP), is one of the three reference pressures the SAAMI performance standards uses to describe the testing of ammunition subsequent to its manufacture, and is a number that has meaning only within that testing environment.
Anyone interested can go to the SAAMI site and download a set of the SAAMI manufacturing standards. Bring your statistics books with you :-)
SAAMI:
http://www.saami.org/
