Hey Beta, If you are dead-set on getting a GBBR AK, I would highly recommend getting the GHK 2013 series. A few quirks here and there, magazines to file and loctite to the threads and you're set. I've been running mine with propylene during the ASG night games and she's been working good for me.
Like what L473ncy said, you should definitely get in touch with Julian (Optix) at Airsoftgear richmond. He's a walking airsoft encyclopedia.