I've got one and they're generally good. Open the mechbox, clean that jelly crap out of there and regrease it properly. or get a gunsmith to do it for you. That's the only issue I've had with my m15a4 and it's had about 8000 rounds through it.
I would get age verified if you're old enough. You can always find an m4 for cheap with extras on the buy/sell section. If you're not 18, you should wait
Cal City - Foreign Branch (Sask)