I know they look cool, but how many real HK11 have you seen with a RAS? If you want to put a vertical grip on your gun you can do for $30. If you need a front end try to get the real stuff and make it work. If you decide to do your own searching you might find more when searching HK91 vs HKG3 or G3. HK91 is a model number and as an import they are commonly referred to buy their numbers. ie HK94 = MP5 (usually with a longer barrel to meet gun laws)
http://www.cheaperthandirt.com/MGR282-1428-985.html
by the way the but stock will work too

A quick change barrel system sounds cool but a lot of trouble. send me a pick when you do it.