The Arc Gun—the most recent non-human weaponry discovery by the Multi-National United—is one bad-ass piece.
It fires a huge charge of electricity to its target, kind of like a bolt of lightening, heating the target enough to cause extreme burns and rapid molecular expansion of gas and fluid. In a soft biological or animal target, an Arc Gun strike can result in a catastrophic explosion. But even a near miss can cause superficial burns and keep an enemy subdued, and bolts landing in the vicinity of a target can cause some area damage as electric current disperses into all nearby conducting bodies.
So with the Arc Gun, the non-humans in District 9 hold the power of lightning in their tentacles. Although most of the known Arc Guns have been confiscated by the MNU to help keep us humans safe, they are one of several non-human weapons floating around the black market...which is basically the scariest thing we've heard in a while. Let's just hope the MNU gets them all.