Floating assault rifles

In 343 Guilty Spark (Level), use the "Grenade trick" to stay in your Pelican (throw grenade before you get out to stay in). If you walk around long enough up top, you can find a Marine, who is competely unarmed. Though I have not found it, that spot is supposed to be full of Flood corpses, and have a Rocket Launcher on the ground.

More importantly, if you go to the point where you descend inside the structure while you are still on top, you can drop to in front of the entrance and see some floating assault rifles firing at the Covenant. The explosions are not caused by frag grenades either, rather they are Spawned effects.