Researcher's letters all date to after the introduction of Western's boomers; 12g ‘Super-X Field’ 2 3/4" 1 1/4 oz. 3 3/4 Dram Equiv. shell in 1922, and the 3" ‘Record’ with 1 3/8 oz. of shot in 1923.
The Double Gun Journal, Winter 2016, p. 129 had a neat article "Ithaca’s Golden Girls” by Jim Foral, featuring Mrs. E.B. Belknap (Alice) and Mrs. Troup Saxon (Ermina)
Forest & Stream, March 1, 1913
https://books.google.com/books?id=mk...J&pg=PA288&lpg
Mrs. E.B. (Alice) Belknap and her No. 4E Flues
Amateur Sportsman, May 1911
https://books.google.com/books?id=m5...AJ&pg=PA36&lpg
Mrs. Troup (Ermina) Saxon and her No. 7 Flues
They were likely shooting 1 1/8 oz. 3 Dr. Eq. loads with modern transducer pressures of 8,500 - 10,000 psi; similar to today's target loads.
And a lot of them.
What John does not know is what loads have been through his Flues, apparently set up for trap, in the past.
But if the frame cracks there is always this
I believe there is much more to the blown up M21 story; just like the "blown up" K80 and Perazzi trap guns. The Model 21 was proved with a “Violent Proof” (VP) load reported to be 7 1/2 (long) tons = about 24,000 psi by Burrard's conversion. One M21 survived 2000 of these loads.