Building one's own sounds very challenging, particularly with respect to the (what I believe to be) press-fit barrel extension attachment system. There's an outfit in Phoenix, Arms Tech, that has made some heavy BARs in 300 Win Mag for the military. They haven't been doing anything with that for the civilian market for a while now, but that could change at any time, I'm sure. See: http://www.armstechltd.com/
