It looks like there is a 20" for you, now how are you planning to get the irons on the end of that barrel? I would like to build up one of these as a trainer but I am not sure how to get the front sight on the naked barrel.
gm
You can use one of these:
http://www.armalite.com/ItemForm.aspx?item=EU0240MPKIT
I have a new one that I was going to put on my 16" upper but decided not to. PM me interested.