If this is your first-and-only rifle, I would definitely go with something in 5.56mm NATO, and with good optics on it. It's the red-dot electronic sight that has given the American grunt unprecedented first-round kills, even in an ambush situation. Surviving terrorists said they were amazed at how fast the Americans swarmed them, and how accurate the rifle fire was, with so many of their comrades in terror dying from headshots.
Get a quality M4 clone, and spend the extra money for an Eotech or Aimpoint electronic sight. This will leave you with about $500 left for ammo and mags.
If you want to spend the whole $2K on the weapon, I'd recommend the SiG 556 SWAT. Sell the mediocre electronic sight that comes with it for $100, and put on an Eotech, like this.
