I personally use the Scar with FMJ and sleight of hand. Sleight of hand solves the low clip size problem almost entirely, reloads are super quick. And while an RDS or holo sight is a vast improvement for most weapons, the iron sights on the scar are about as perfect as can be. If anything, the RDS or Holo are a downgrade on it. The iron sights are super thin, and you aim down them faster than the electronic sights. With sleight of hand pro, its practically instantaneous.
Add stopping power to the sleight of hand, and you can sight and take down anyone a split second before they even get a shot off, at any range. The scar is dead accurate, and it only takes 2-3 shots to kill even at a long distance.
Im working on getting the extended mags, which will probably replace FMJ for me - I'm assuming these are larger clips, and not just a few extra mags in reserve. Its quite a bit harder than it sounds to get 40 FMJ bullet penetration kills though...going to take a while.
Besides the scar, I've been using the F2000, just unlocked at 60 - I really dont like it. Its more like an SMG than an AR - the rate of fire is absurd, and it actually has a lot of recoil. The TAR is inaccurate, but its more of a spread. This thing actually kicks upwards a significant amount. I've gotten quite a bit more headshots than usual due to the upward kick, but recoil + weak bullets is generally not a good combination for anything but medium to close range.