Hey man, I can't tell you anything that hasn't already been said. However, I do urge you to take a look into the VFC M4 Sopmod, I picked it up recently from Army Issue (who had it ordered in from Mach1 airsoft). It came to around $680 with taxes but since your budget is $500, it's really not that far off. The VFC M4 is my first airsoft gun and it truly feels and looks amazing, there's a very good review of the gun on this forum that is easily tracked down using the searchy thingy. I will also mention that this gun was pretty easy to disassemble and reassemble (reassembling was tad bit difficult at first but that's because I'm slightly retarded). I mention that because it's really important to me as an airsoft newbie to figure out how to maintain a gun and it's so much easier to do this when I'm not intimidated to open my gun. Good luck and happy hunting!

I'd take nylon over pot metal any day. It'd be tough to choose between aluminum and nylon though. I love my f2000, but crappy plastic is waaayyy worse than crappy metal :P

Depends on the plastic. If it' nylon or some other strong plastic composition, I would take that over metal. But, crap ABS plastics and whatnot have a tendency to break after a while. I've gone through about 3 plastic lower receivers with my G&G M4 Blowback, but that's with a lot of abuse (going through over 50,000 rounds + lots of dropping).

The G&G Plastic receivers are not too shabby if you're careful. But IMO if you plan on modifying or fielding your gun a lot, I would go metal.