Floating assault rifles

Note: This has been tested and confirmed on 343 Guilty Spark (Level).

On a level with a Pelican, such as 343 Guilty Spark (Level) for example, it is possible to stay inside the Pelican. When you come close to being dropped off, throw grenades. If it works, you should stay inside the Pelican.

Path to Get There
After you get off the Pelican, quickly run to the area in which you go into the structure with the Flood. If you walk around long enough up top, you can find a Marine, who is completely unarmed.

Floating Assault Rifles
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. This saves a lot of time instead of going through the entire swamp. If so you choose, you can get back on the Pelican in the level Assault on the Control Room by pressing "X" when you are dropped off. You can also get out after the Pelican takes off again, and will be offered a chance to flip the pelican using "X" before you fall to your death.

Alternative
An easier way to get to the unarmed marine and floating assault rifles, is to 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. Wander around for awhile, through the 2D trees,(i believe to the north west displayed on your assault rifle your given but i could be wrong) until you find the marine. (Sometimes he will say things when you appear, like: "sir!" or "You scared me sir and now i've... soiled myself." or sometimes even "Jason Jones is the greatest man i've ever seen!" Keep walking away from him until you see the base with the assault rifles and jump down, look into the base and there they are! shooting at the covenant.

NOTE: This alternative will cause you to skip half the level, the assault rifles can hurt you if you stand in front of them.