There are several factors which come into play here. As the chamber for the rifle cartridges are smaller you are adding extra metal around the cartridge to handle the higher pressure in the barrel. Back thrust to the action is a factor of area times pressure. As the rifle cartridge head is smaller thrust is not just a factor of PSI's.
Another factor, just for the record, is you can "Only" make a monoblock from Scratch, you cannnot make one by cutting off an existing set of barrels. "MONO" = 1. A Monoblock is made from a single piece of steel. Neither a chopper lump, dovetail or other methods of jointing can be converted to a monoblock.
