I think your comparing apples to oranges.
I shoot LOW PRESSURE 2-3/4" shells in my short chambered guns.
I think a 3" magnum 12 gauge load is already pushing the upper limits of SAAMI max. pressures. So, you would stand a pretty good chance of pushing it over the PSI limits by shooting it in a 2-3/4" chambered gun. However, it's your gun.