Hi, we as you know have no National Proof House, as they have in most European countries. There is an organization that most American Gun makers belong to. That organization is:
The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI). Their site is:
http://www.saami.org/specifications_and_information/index.cfm
While guns may be "proofed" at 22000psi, that is not the safe or recommend psi for standard service pressure. A gun is "proofed" using not more than 2 proof loads normally. The following is the standard recommended service pressure for modern loads:
ANSI/SAAMI Shotgun Pressure Specifications (Maximum Average Pressure in PSI)
All Piezo measurements unless specified)
Cartridge Maximum Average pressure
10 gauge 11,000 (all)
12 gauge 11,5000 (all but 3 1/2" mag)
12 gauge 3 1/2" mag 14,000
16 gauge (11,500 (all)
20 Gauge 12,000 (all)
28 gauge 12,500 (all)
.410 Bore 2 1/2" 12,500
.410 Bore 3" 13,500
I assumed that the OP is referencing a 12ga gun, but as mustistuff rightly said, the smaller the bore the higher the pressure.
Pressure can be a very tricky deal, always better to err on the safe side. If your gun is modern, you'll have no problems, but if it's an older/vintage gun, things get a bit sticky.
Good Luck!
Greg