Note that even if it is chambered for 3 inch shells the 20 ga "3 inch Magnum" as we know it today did not exist. 3" 20 gauge shells could be obtained on order but they were normally loaded no heavier than 1oz of shot.
Also the fact that a 2 3/4" or 3" shell will go in does not mean it is chambered for them. The length of shotshells is determined by their "Fired" length. A loaded shell with fold crimp will enter a much shorter chamber than its designated length.
You do not have the same situation with shotshells as for instance that a .357 Magnum pistol cartridge will not enter a proper .38 Special chamber etc.
