The rim of a 20 ga shell in a 16 ga chamber will stop within the rim recess of 16 ga chamber. On my Model 12, 16 ga the base of the shell is just below the rear of the chamber ring. On a Savage 311 the shell drops in about 1/16" below the breech face well beyond the reach of the firing pin. If either gun were in a muzzle up position and fired it may be possible for the shell to fire depending on the primer sensitivity, pin protrusion and length and spring strength. It is not possible to stack a 16 ga on top of a 20 ga shell as in the 20/12 scenario. Jim
reason im asking is a fellow appears to have loaded a 16ga with 20ga hulls and was firing away with no issues. he was shooting an old lc smith. i never got close enough to see if it was a 16ga. issue was his hulls were slipping past his ejectors and chambering. im thinking now it may have been just a worn 20ga gun.