My first 3 guns gold were Vector, Assault Shield and KSG.
Just do core and for bloodthirsty, Silencer and some sort of concussion w/ scavenger. Keeps everyone away enough to shoot and run. Pretty easy since most of the time your 1 bullet will beat the enemies 2-3.
Edit: Remember that the Silencer reduces the 1hit kill range by a pretty big gap compared to long barrel..
Using KSG in hardcore seems counter productive. Why take a gun that's a one hit kill in core (a significant advantage) into a game mode where all guns kill in 1-2 bullets?
This. I remember going from a Core match where I was destroying with the first shotgun, to a Hardcore match and not even getting a single kill. Being able to kill in one hit up close doesn't mean anything when everybody can kill you from pretty much any distance just as fast.