OOOOOOOOOOOOOOh... to be honest I've been neglecting my ASR recently, too much focusing on other projects and not enough games to baby it recently.
But, for that screw I would first try one of these
Called an extractor or extraction bit (what Mgunner was talking about) you drill a hole down the center with a normal drill bit and jam that other thing in there and back it out.
If that doesn't work I'd take a my Dremmel with a small cutting disc and make a deep cut perpendicular to the barrel and use a blade screwdriver. You'll probably hit the plastic a little, but not enough to really damage it.
Edit: as a side note, this same thing happened to the Aero chamber on my ASR, I was able to go to the local ACE with the hop up in my pocket and find a chrome molly metric hex screw to fit in there; no more stripping problems! And now I can take down my whole ASR with just a set of hex wrenches
