mossbergman11

If you look at the end of the forearm - actually the end of the magzine under the barrel there should be a pin that you can push/pull out. When you have that pulled out and the barrel insterted into the receiver you need to rotate that pin to lock the barrel, magazine into the reciever. Then push the pin back in.

Let me know if this made any sense - If not I will go get one of mine and take some digital photos...

Check here for a parts blow-up

http://www.e-gunparts.com/productschem.asp?chrMasterModel=0950z12

Part #52 Magazine locking pin is what you are looking for