Sporting Guns and Gunpowders: Comprising a Selection from Reports of Experiments, and Other Articles Published in the "Field" Newspaper, Relative to Firearms and Explosives, Volumes 1-2, 1897
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA296&dq
Pressures were measured using crushers (LUP) and reported in pounds/ sq. inch; modern piezoelectric transducer pressures would be 10 - 14% higher.