AirsoftNT in Hungary shows how to change the hop-up rubber of the Double Eagle M904... "I made a video about a basic development for any airsoft replica - changing the hop-up rubber. A green G&G rubber will be inserted into the Double Eagle M904 purchased earlier, there will be a chrono and test with different BBs.

IMPORTANT: This video is specifically for the Double Eagle M904 series, but the procedure is completely similar for most AR15/M4 platforms."