I disagree about Cherry + Art of war at the same time, at least for assault rifles. You'll get more kills much more easily doing one then the other with a 6 perk class / 3 attachment & tactical class over 300 kills than you will with a basic gun over 150, not to mention that you'll get alot more doubles / bloodthirsties as you do it too, and probably won't harm Kd/spm so much if you care at all about it.
I just finished the SWAT-556 not too long ago. My KD with the SWAT-556 rose from 1.58 to 1.71. The difference between my games with the FAL OSW and SWAT-556 was my level of aggression. I was way too aggressive for having such a gimped class. I had similar experiences with the other assault rifles I've gotten gold, although my KD with those didn't rise nearly as much. With the SCAR-H, it rose from 0.96 to 1.17.