Scroll down just past 1/3 here for a summary of “Long Shells in Short Chambers” by Sherman Bell with technical assistance from Tom Armbrust in “Finding Out for Myself” Part V,
Double Gun Journal, Winter 2001. AFAIK this is the only pressure test conducted and published.
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/editOne load had a 1200 psi increase, which might not be "perfectly alright" in
your gun, and would emphasize the importance of following Bell's advice:
“Shooting 2 3/4” shells in 2 1/2” chambers does make them produce more pressure-but in most cases it is less than a 1000 psi increase. I see no reason, related to safety, to modify an original 2 1/2” chambered gun to shoot 2 3/4” shells,
if the 2 3/4” load you intend to use would develop pressure that is safe in that gun, when fired in a standard chamber!”
Of interest, but obviously different shells
“The Long 20 Bore Cartridge”,
The Field in
Forest & Stream, October 17, 1908
https://books.google.com/books?id=ejQevDPMUIYC&pg=PA630 2 3/4” case with “Schultze” Bulk Smokeless (42 gr = 3 Dr. Eq.)
The standard 2 1/2” case was loaded with 33 gr. “Schultze” = 2.36 Dr. Eq. (about 2 3/8) with 13/16 oz. shot.
36 gr. = 2.57 Dr. Eq. with 15/16 oz. = 4.65 tons =
14,504 psi (using Burrard’s formula) / 20 yd. velocity 1000 fps
37 gr. = 2.64 Dr. Eq. with 7/8 oz. = 4.54 tons = 14,134 psi / 1040 fps
35 gr. = 2.5 Dr. Eq. with 13/16 oz. = 3.45 tons = 10,472 psi / 1068 fps
