Whatever you want! There are only two rules: it must be possible using the technology of the Halo universe, and it must be light enough to be carried by a SPARTAN-II or Elite.
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nerve gas gun. sprays a high concentration of nerve gas in a controlled stream of pink death! Used on Covie homeplanet where aliens don't wear armor. (Mainly covie civilians, cuz militant covies still have armor and some carry their own environment, so they live) raises hell with marines, too... hehehehehehe