You'll find on the internet < 8000 psi, without evidence as to where that number originated.
Possibly with this
Forest & Stream, November 24, 1894. “Gas Pressure or Bursting Strain”
https://books.google.com/books?id=19kwAQAAMAAJ&pg=PA459&lpg The proof-house charge of the American Testing Institution for a 12-bore gun develops a gas pressure of over 18,000 lbs. to the square inch. It is the same strain to which finished guns are put abroad. No gun would stand such a strain for any length of time. About 10,000 lbs. may be considered of touching the safety limit, and for some guns even is hazardous.
A load giving about 8,000 lbs. pressure and less is better suited for the average gun and everyday use.
1 1/8 oz 3 Dram Bulk Smokeless loads at the turn-of-the-century did run < 8000 psi. Dense smokeless about 1000 psi higher.
