The rearward thrust, called bolt thrust in bolt actions, is the product of the pressure times the surface area on which it acts. In the case of cartridges, the force we are interested in is that acting in the rearwards direction. It is the product of the pressure times the surface area of the inside of the rear surface of the cartridge ie not the surface area of the external dimension of the case rim. eg a 45-70 may have a rim diameter of .608 but the internal diameter is considerably smaller. It become important when someone claims that because something like the Baikal MP221 can take 30-06 pressures therefore you can use similar pressure loads in the 45-70 variant OOOPS.

As detailed above the locking mechanism on doubles also involves other torquing factors which means the end result isn't so simple.