OK, so, first, I'll describe my brief foray into the world of sniping. All Ghilied up is an excellent mission to test out your scope settings, and I set mine to the Professional preset, with just a few differences here and there. I also switched my main reticle to a plain white dot, even with bigger guns. I've gotten better kills this way, even without lag.
So, I tried out the two rifles I had at my disposal, the M40A3, and the M21. The M4, I absolutely hated. It reminded me of why I only used the PTRS in WaW (cause that was arguably the most semi-auto Bolt-Action Rifle I've ever used). Sure you have more power than the M21, but if you aren't a stellar shot, you're screwed. The M21, I had better luck with. I am about a fourth of the way to the M21's ACOG, but I've been shotty-running as of late, so I haven't budged for a while on the M21. I'm a fair sniper with a semi-auto rifle. One round I got a 15 kill streak with the M21 on Bog (the taking out the entire enemy team with my airstrike and then calling a heli on them didn't hurt either). Then some fool shot a n00bt00b into the side of the bus, and that ended there. I also tried the Dragunov, and I think I could get used to it after I get my ACOG for the M21.
Next, I tried Shotties. Oh. My. God. W1200 + Grip + Juggy + Steady Aim = RAPE
The only hard part about shotties is learning your boundaries. Once you get those down, you can be a potent force of destruction, and stuff. My favorite thing to do is pseudo-Ghost while shotgunning. I'll go up behind an enemy sniper, and then give them some buckshot to the back of the head, and then evac. Only thing is, then they get pissed. Meaning you have to stay in the shadows, or else.