Help identifying black ops gun. ?

I play black ops online and I have noticed there is one gun that always seems to kill me. When I notice it, nobody is using red dot or any other scope, they are just looking down the sights. It is fully auto and silver, any ideas? It appears to be very accurate and pretty powerful, I may have already unlocked it but I can't seem to find it. Also, what is the best set up to use? I'm talking primary, attachments, secondary, perks, everything. All ideas welcome.