i have the same probleme with my echo 1 m14 made by cyma...
in this picture it show you the cut on the barrel for the hop-up unit (the text is part in french) the 6.01 is a madbull and wont fit due to those cut.... so i must find someting else too. any idea???
i'am waiting a confirmation for the fitting of this one... from their tech.
http://www.evike.com/product_info.php?products_id=40987