SCAR-H with a grenade launcher or P90s with Akimbo and Rapid Fire.
I assume you're tossing Steady Aim on your P90s, eh? My problem with Akimbo is that you lose your ability to fight over range. You're far better slinging rapid fire on it, right? Then you still whipe people out instantly, and you aren't a sitting duck if someone is 50 feet away. I gotta say though, akimbos are nice if you never hit open ground or you're fighting on Rust.
The SCAR-H is the first weapon I worked to get all of the attachments on. It's a thing of beauty, with that damage and accuracy. However the clip size is cripplingly small and I have much more success with a heartbeat sensor or a scope than the noob tube. Of course the noob tube is a guaranteed kill or two if you get the chance to fire it, but once you have enough skill a scope or sensor will net you a spree rather than a single kill or so.
What do you like for secondaries? I use AA12 on assault rifles, launchers or AA12 on submachine guns, and machine pistols on sniper rifles. The only machine gun I ever touch is the AUG HBAR, which is an assault rifle on roids, and I keep a stinger on that to annihilate anything in the sky. Heartbeat sensor, Grip, Bling, Cold Blooded, Ninja, Flash, Semtex.
My best success is sniping with the Barret, and I don't see any purpose in all of the other sniper rifles. Do they have any advantages or any differences from each other? (Aside from the obvious bolt-action slow fire of the Intervention). I get from 3 to 11 kills with it and the machine pistol secondary with ease; it's almost too easy. Stopping Power and moving around beat trying to hide with Cold Blooded unless you play Hardcore. I don't bother trying to hide my sniper; if people come looking for you it's just another easy kill when you bushwack em with the machine pistol or claymore.