I believe Ducota is referring to the different machining between 20 ga sized shotgun and rifle actions. In a rifle destined action lateral walls are much thicker and the bottom is not hollowed out, making for a much sturdier and heavier action. Of course, subsequent heat treatment is also adapted to the higher rifle pressures. To illustrate the point, here's a pic of my rifle action (R) next to one of my B25 shotguns (L):

Although the shotgun is a 12 ga, the comparison with the "smaller" 20 ga sized rifle action still points to the latter's thicker walls and full bottom devoid of lightening cuts. One more difference is the presence of hollow gas screws in the right side of the action to provide a lateral escape route to gas, should a case burst or a primer leak. One has to hold the rifle in hand and look very closely to locate the screws for the screw heads are covered by the engraving.