you need a modified upper and bolt carrier you can get both from
http://www.mgimilitary.com/store/ind...eab34caaedb8a9
http://www.mgimilitary.com/store/ind...products_id=16 there upper is costly but already mod. or you can send your upper n bolt to michael machines and he will mod. is for 100.00 his email is
hkmp5@empowering.com