Did a little searching and found this link. Looks like anybody with a little common sense and a good belt sander and dremel tool could do it.
http://www.airsoftsniper.co.uk/index.php?option=com_content&task=view&id=39&Itemid=26
http://www.airsoftsniper.co.uk/index.php?option=com_content&task=view&id=39&Itemid=26