Barrel help

[MagShootinElmo]

I have a vert feed e-mag and i am aware that i need a special barrel for that but no matter what site i go to they dont ask vert or power feed, where can i go to find whatever i need to put a barrel into my mag

[killerbombo]

Smart Parts offers Freaks for it, but only in an aluminum back (no stainless). I don't think they list it, but that was there response when I e-mailed them about one a week ago.
Regards, Mark