They have grimoire cards explaining all about each enemy race's armaments, yet we can't play with them. Why not? How 'bout we just use what ammo is in the clip then the gun auto-drops? Why not get to use them as a special weapon drop in the Crucible?
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Because this isn't halo, the enemies don't drop their weapons, instead they are coded to drop glowing balls the cryptark uses to troll you.