I have a TM upgraded hop unit and a TM stock one. The stock one is in my VSR and it works well but when I try to put the upgraded one in it then the bbs will not feed unless I push on the bottom of the mag. I haven't the foggiest idea of what could cause this. I even had an old VSR that I bought used which the upgraded unit came with and it worked perfectly in it. Also for that matter what is the difference between the stock unit and the upgraded one? They seem to be exactly the same in every way except the color.