I have both G&G combat machine and CAA (asg) M4's, the G&G has a metal buffer tube the CAA doesn't, neither is the bolt dust cover, mag release, hence why it's the sportline I guess, the gearbox was by kings arms, the hand gaurd and the butt stock are by CAA which are a nice solid polymer, the receiver is also polymer and has CAA trades on it, I was lucky enough to have the extra parts to replace the plastic parts, the CAA also comes as a full metal version as well. Straight out of box I would go for the combat machine as it comes with metal buffer tube and mag release, if want to go down route of changing buffer and mag release, cheap enough if you shop about, then you don't are worry about a nice looking stock and handguard.