Forge Turret Glitch
From Halopedia, the Halo wiki
Were you looking for Unlimited Dominion Turrets?
The Forge Turret Glitch is a term used to refer to two separate glitches in Halo 3, Halo: Reach's, and Halo 4's Forge. The first glitch, which allows players to duplicate turrets endlessly, is the result of an unanticipated design flaw that allows players in Edit Mode to manipulate turrets while they are in use. The second glitch allows players to bypass thin barriers using turrets.
Start a Forge session with two players, Person A and Person B.
Person A should enter Edit Mode and spawn a Machine Gun Turret or a Plasma Cannon, and then set the turret down. Person B should take control of the turret. Person A should, at this point, grab the turret again, lifting it and Person B into the air. (Person B can also start opperating the turret while Person A is holding it.) Person B should now press the "Melee" button (default on MCC is B), to tear the turret off of its base. The duo will find that Person A is still holding an intact and usable turret, while Person B is running around with a detached copy of that turret. At this point, Person B can discard their turret and repeat the glitch to create another copy, or a hypothetical Person C can repeat the glitch (with C mimicking B's previous actions) to get their own turret. This glitch also works in Reach and Halo 4.
This glitch can be done with a single player.
Place a Machine Gun Turret against the wall you wish to bypass. The Turret should face away from the wall (exactly 180 degrees away), and it should be placed as close to the wall as physically possible. Enter Player Mode and man the turret. Look as far upward as you possibly can. Depending on what kind of wall you wish to bypass, the camera may move behind the wall. When it does, enter Edit Mode, and you should find yourself behind the wall. If you don't end up behind the wall, try dismounting the turret right before entering Edit Mode. This glitch also works in Reach and Halo 4.
A two-player method, similar to the Duplication method described above, allows players to bypass vertical surfaces.
Have one person (Person A) spawn a Machine Gun Turret and remain in edit Mode. Have another player (Person B) man the turret. Now, have Person A lift the turret and take Person B to the desired vertical surface. Person A should rotate the turret so that Person B is in between the turret and the surface, looking away from the surface. Person B should, at this point, enter Edit Mode.
This glitch's uses consist primarily of bypassing barriers and entering areas that are normally inaccessible.