Sorry Will. Bell did not report the psi of a factory 20g AA load, in a 20g chamber, down a 20g bore - not using the adapters.
Actual testing by Tom Armbrust showed the 12g 3 Dr. Eq. 1 1/8 oz. Winchester Trap Load to run at 1,202 fps with a pressure of 9,600 psi. I haven't found a 20g report, and the factory won't tell you beyond 'it's within SAAMI specs'