Go to http://www.subguns.com
and then click on the machine gun discussion board and ask your question there. Two top notch HK gurus are Stan Andrewski and Terry Dwyer. I don't have their contact information handy but the guys there will help out. If not post back and I will dig up the info for you. The only problem with these guys is that they are backed up on work from usually six months to a year.