Many many combos my friends, heres the rundown:
Close range-
1)Magnum + SMG (get some headshots with a mag, and clean up with the SMG
2) Plasma rifle and the SMG (use a powered up Plasma blast, and finish with a spray from the SMG)
Medium Range-
1)use any combo of Plasma rifle, magnum, and the SMG.
2)Plasma pistol and the SMG
Long Range-
1) Duel magnums with crazy good aim make good long range weapons.