When I was playing on Rek I ran a lot of L4s for Sebiestor Tribe with a 1400mm Tornado. With 90km or so optimal+falloff with the "high damage" ammo types, very few rats will be able to touch you. And the alpha is just incredible, though I fired mine in four groups of two guns in order to counter repping rats and to land critical hits more often. The Tachyon Oracle and 425mm Rail Naga are also decent for the same purpose, though with the former you definitely must degroup the guns as described above and stagger fire in order to keep yourself from capping out while running that MWD.