Try airsoft buddy. I'm 90+% sure the M4 and the SCAR take the same types of barrels (i.e. the cuts/notches and attachment to the chamber are the same)....I'm not 100% only because I haven't actually done it myself.
262mm
http://www.airsoftbuddy.com/index_ep...roducts_id=479
363mm
http://www.airsoftbuddy.com/index_ep...roducts_id=478
505mm
http://www.airsoftbuddy.com/index_ep...roducts_id=480
Haven't picked it up yet, but my hopup rubbers and 363 barrel came in last week.