Hi,I am trying to remove the standard stock on my m4 to replace it with the skeleton collapse stock.I pushed the pin out and removed the trigger assembly but how do I get the stock off.I tried turning it but it wont budge.Do I have to loosen the nut through the back end recoil pad or what?My model number is 11720(L.E. version with standard stock).Will the collapsible stock even fit this model?? Thanks for any help.My email is;[email protected].