Very interesting. We've discussed the 1929 Hunter Arms Proof Load Pressure Curve Chart in the past
http://library.centerofthewest.org/cdm/singleitem/collection/WRAC/id/8149/rec/107

12g 2 3/4 was proved with 6.5 Drams FFFg with 1.687 oz. shot = 14,200 psi at 1"
Pressure was about 9,500 psi at 6"
Pressures were measured by crushers (LUP - Lead Units Pressure) and modern piezoelectric transducer measurements would by 10 14% higher