I finally got a Rock River M4 16" with full quad rail a couple of weeks ago. It feels like a toy compared to a standard AR-15, but man is it fun to shoot.
4 30 round and 2 20 round Promags are loaded and ready to rock. Next up on the list: 24" AR-10 .308 or Remington 700. Any suggestions on reaching out past 500 are welcome, though.