It should also quite well be noted the 2 3/4" shell was not introduced with the "SuperX" load. In fact drawings fron 1907 found in The L C Smith Plans & specifications book show the 2 3/4" shell as already being standard for their 12ga guns. Virtually all of my post 1900 Lefever's in 12ga also have 2 3/4" chambers. I of course cannot prove they came that way from the factory, but I also have no reason to believe that "Everyone" of them have been re-chambered. In fact the big "Theory" behind the introduction of the SuperX loads was the newly developed Progressive burning powders allowed higher velocities with a heavy shot load "Without" a resultant increase in pressure. Since, as correctly stated, we have no real record of pressure prior to the introduction of SAAMI, I think it totally & absolutely "Self Serving" to infer that all loads on the market prior to that time were "Low Pressure". In fact I do not believe there is a shred of proof the previous 7/8oz, 1200fps 20ga loads the Ithaca Flues had been digesting quite happily had a bit lower pressure than did the new 1oz, 1220fps SX loads which cracked frames. They did however have a heavier "Back-Thrust" to the head of the hull. I am old enough to remember when further powder developments allowed the introduction of the so called "Baby Magnums" in the 50s when standard length shells had their charges increased to 1 1/8 oz in 20ga, 1¼oz in 16ga & 1½oz in 12ga. As these shells were publically stated to develop no higher pressures than had their predecessors many assumed any gun they had been firing with the older SX's were quite suitable for these newer magnums, this was not always the case, but again not because of the higher pressures but the heavier back-thrust. Fortunately we have a readily available formula for comparing this "Back-Thrust, its called recoil. Given the same gun you can bet if a "New Load" kicks harder then not only "You", but also all the parts in the gun which resist the axial loads present from firing the gun are being put under a greater stress.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra