Floating assault rifles
From Halopedia, the Halo wiki
In the levels Assault on the Control Room and 343 Guilty Spark on Halo: Combat Evolved and Halo PC, it is possible to see some Assault Rifles floating in mid-air. They are known as Floating Assault Rifles.
Assault on the Control Room
The first floating Assault rifle can be found on Assault on the Control Room, in the first room where Rocket Laucher rounds can be seen. After you kill the Elite that has his back turned on you, move forward and before the next door, there will be an Assault Rifle, Assault Rifle rounds, Pistol rounds, a Health Pack, and Rocket launcher rounds. While you pass above them, look at them. The Assault Rifle will suddenly rise and float in mid-air. It can still be picked up and its rounds can be picked up too.
Note that this glitch is sometimes hard to see, and you may have to play the level on a specific difficulty level to see it.
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343 Guilty Spark
Method One
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 MA5B Assault Rifle fire coming from inside a structure. Go around the level area and you will eventually find yourself above this structure. As the firing begins, jump down and enter the structure 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 a lot of time instead of going through the entire swamp.
Method Two
An easier way to get to the floating Assault Rifles, is to either: Go to the front of the Pelican, and 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 stay in the same spot. Keep moving around the Pelican so you don't fly off. Wait until the back wing 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.
Method Three: BEST METHOD
When this level is loading (PC or XBox version, it doesn't matter), hold down the grenade button/trigger all the way to and through the opening cutscene. When the cutscene ends, Master Chief will throw a grenade out the back of the Pelican. You can release the button/trigger when he throws it, and instead of getting out of the Pelican he will stay in it as it flies away. Then the Pelican will fly away the same way it does in Method One, and eventually it will stop and you will automatically jump out and you will be on top of the map where you can easily get to the entrance of the complex where the Floating Assault Rifles are.
