I believe they are all tapped there should be four plastic plugs in the top of the reciever. You have to disassemble the gun an push these up from the bottom.
The only time I have expierenced this is while dry firing when not holding the fore arm. When actually shooting the weapon I have never had this happen.
I have a brand new supernova with pitol grip in black. Would anyone be interested in trading a regular stock for my pitol grip. I am located in central illinois.