If you will check,Olin had developed the "Super-X" before he bought Winchester. He wanted to buy Parker, but Remington did so first. So, he used Fox as the vehicle to use his new shell, a Western Cartridge Co.(which he owned) shot shell.His engineers had used high speed photos to analyze shot as it leaves the barrel. The opinion was that gas leakage would be reduced if the paper of the shell extended into the forcing cone.When he bought Winchester, the knowledge was used and although the M-21 is marked 2 3/4, the chamber is slightly shorter (about 3/32").Check me. I own several M-21's in 12 ga. and only found out 15 years ago when I bought a pair of Briley 28 ga. tubes for pigeons and they would not fit. Thus the research. It is on the net. Go find out for yourself.
