Aluminium receiver in the hk roller locked series is bad. The trunion is supposed to be welded into the receiver for strength. With the aluminium receivers you can't do this so the trunion is pinned and epoxied in the front of the receiver. Basically they "fall apart" when the trunion loosens up with use. That's the reason Springfield Armory gave up on fixing the G3 clones they built on aluminium receivers and started offering credit toward M1As.
Also the cocking tube is welded to the triple fame b/c of the aluminium recevier, it's not supposed to touch it, the barrel is supposed to be free floated. The one I had got didn't even have the hanger for the handguard, handguard was just pinned and left loose.
Most people agree they might be good as a G3 parts kit, and depending on what you paid you could be ok. G3 parts kit aren't really cheap. I bought one for $300 with ammo from a guy I knew. Kept the ammo and traded the FA91 for a WASR; I still wanted a G3 clone so I bought a PTR-91 a little later (much better rifle). The FA91 worked ok with the few rounds I put threw it but I knew after looking it over there was no way it'd last.
