Turret detach glitch
From Halopedia, the Halo wiki
The Turret Detach Glitch is a glitch that only works on Halo 3. While trying to detach a turret, let go. There should be a turret in your hands and and one still on its original place. This can also be performed in Forge mode. Simply use a turret as normal, and then get a nearby player to turn into the Forge monitor. Have them move you around and press B to detach. You will then have an equipped turret and the ability to use the normal turret as usual. This can also be achieved by simply using the turret and breaking the turret off normally in Forge. This may or may not work on all levels. Another way to do this is to turn on indistructible vehiles and just pull it off reguarly[hop on it then press B].