Ok, please don't flame me becauce I know nothing about AR style weapons. Here's my question. I have a complete HK416 kit . When I try to install the HK hand grip on the receiver, Either the web on the receiver is thicker or the slot in the grip is smaller (I have no other AR parts to try at this point) I had heard that all AR lower parts were supposed to interchange with the 416 lower parts. Has anyone had this problem?