another thing which might be influencing your gameplay could be your connection - while you in a game just hit start(ps3) or back (360), and it will come up with the leaderboard. Find your name, and look to the right of it, at the end. If there is only a little red bar, your probably lagging heaps. If green/ yellow, its not that bad.
My suggestion for a class which i once got 123 kills for 11 deaths:
MP40
3x gas
stopping power (bullet damage)
steady aim.
For core, it kills most people in 2 hits, even with juggy
For sniping, stick with the third sniper - i forgot its name - with bullet damage and steady - it goes hard.