Floating assault rifles
From Halopedia, the Halo wiki
It should be noted that unlike the floating plasma grenade glitch, the actual presence of floating MA5B assault rifles is not a glitch. The fact that the player sees them is the glitch. Bungie added the floating assault rifles to imply a Marine or Flood presence inside the structure that was scaring the Covenant troops away. The most likely reason why the rifles are floating instead of being wielded by non-playable characters is that the NPCs would have built-in A.I. personalities, causing them to move around and spout combat dialog, thus alerting the player to their presence. The fact that there is no combat dialog or movement of the rifles adds to the mystery of the level.
Use the "Stay in the Pelican" glitch to remain in the Pelican that drops you off at the level's start. Remain seated until the Pelican comes to a full stop at the top of the level. After you get off the Pelican, quickly run to the area in which you go into the structure with the Flood. When playing the level normally, you would eventually see several Covenant running from assault rifle fire coming from inside a structure. Head towards the structure while remaining on the high ground. When you are nearly above the structure, jump down in front of the entrance, and you will find that no one is holding the assault rifles. The explosions are not caused by frag grenades either; rather they are spawned effects. This saves time, instead of going through the entire swamp.
For those that like a challenge, exit the Pelican normally and head towards the front of the Pelican. There should be a big rock. Jump on the rock, and from there jump onto the nose of the Pelican (do this quickly before the Pelican flies away). Now try to stay in the middle of the top of the Pelican because as it flies away, your character will not always stay in the same spot and be pushed off by the physics engine. Keep moving around the Pelican so you do not fly off. Wait until the aft nacelle is close enough to the cliff nearest to the Pelican, (this will take a few tries so just keep respawning back to your last checkpoint if you fail) and crouch jump onto the cliff. If timed correctly, you should not slide down the cliff, and can now walk around the top of the level.