Rab use the M16 for now.

And once you get to rank 6 (I think) you'll unlock Barracks, inside are Challenges for for certain guns ect. Keep plugging away with the M16 and after 25 kills with it you'll unlock the red dot sight which makes it alot easier to aim. The M16 is the most unbalanced gun in the game, it has basically the range of a sniper and is usually 1 or 2 bursts to kill someone.

Perks wise, use Deep Impact and Stopping power untill you get the hang of it.

Either that or use the SAW and just keep your finger on the trigger