Also, read up on how to lubricate the AR 15. RUN IT WET. Like, really wet. Like, when you think you have enough lubrication on it, put on more. It will run dirty, and it will run forever as long as it is wet.
CLP, Froglube, Slip 2000, and 10w-30 motor oil are all great. Just use it generously on the bolt carrier.