I used the FAMAS until I unlocked the TAR-15 (like level 22?). Once I unlocked the M16 (40) until I got the ACR. At 70 though, nothing beats a silenced AK-47. I love that gun so much. I debated not doing prestige just because of it. Oh well, with the stupid glitch I can chopper gun 9v9 on rust my way to 10th prestige.
None of them use slugs though. They all shoot a spread and every shotgun except the SPAS, M1014, and possibly the Striker shouldn't be in the game. They are game breakingly over powered. AA-12? Fully auto shotgun? I challenge any of you to shoot a fully auto shotgun and remain standing after 8 shots. Akimbo is another thing that totally destroys the game. Nobody holds two of any weapon efficiently enough to hit anything. Especially shotguns or SMGs.