As mentioned by thephenom it isnt really fastend onto the barrel rather it sits on it. So yes it does move around but I've noticed no ill effect because of it. I cant remember how it was on the TM m4a1 but I don't think it moved at all. The Picatinny rail can be found on ebay etc , mine was bought from marstar canada who sell real steel accessories and guns .
If they take away the toy guns from law abiding citizens, sooner or later, only the criminals will have the really neat toys.