The Type 81 receiver is probably much stronger than its AK counterpart, considering the same thickness in the stamped metal, due to the two-bend taper that runs from the front to the back. It certainly won't flex as much.The RPK's (Type 56 LMG) receiver is thicker than the AK's (Type 56's) I wonder if the Type 81's is - or if they did what they did to our AK's with longer barrels, and just built them all on the LMG receiver flat.
Whether or not that extra rigidity is enough to just ignore the increased stress of the LMG is another story. I think so, but I obviously have no proof.