Major Sir Gerald Burrard,
The Modern Shotgun, Volume II, “The Cartridge”, 1955 3rd Revised Edition, p. 154 in reference to modern “star” crimped paper cases
“For all practical purposes any increase in pressure due to the longer cartridge case really does not exist provided the correct powder and shot charges for a nominal 2 1/2 inch cartridge are used.”
Obviously this applies to 2 3/4" hulls in 2 5/8" 12g chambers.
A summary of Bell's and Armbrust's study “Long Shells in Short Chambers”, in “Finding Out for Myself” Part V, Double Gun Journal, Winter 2001 is about 1/3 down here
https://docs.google.com/document/d/1...vwLYc-kGA/edit
The pressure increase for one load was 1200 psi
OTOH: I had a 1924
20g Parker with chambers slightly longer than 2 3/8". Recoil with standard 2 3/4" 7/8 oz. skeet loads was severe, and the case mouths were feathered, evidence that the hull had entered the forcing cone.
Hybrid Mode