The freakin cerebral bore from Turok2!! That gun was awesome!! But with Halo, you should be able to see the little sphere thingy that it shoots and the two little hooks that dig into the alien's brain and you can see all the brains and blood and -blam!- bursting out the sides of it's head. Then it explodes, and the alien falls down (without a head). And the little hooks are powered by some kind of futuristic energy that lets them smash through shields and armour (elites and hunters). Maybe some kind of forerunner gun that you find in Halo2? That would be freaking sweet.