Spinning gun
From Halopedia, the Halo wiki
In Halo: Combat Evolved, Bungie's weapon modelers chose to put the pivot point for their weapons at the center of the gun. Because of the way Halo's physics engine works, this means that, because a round fired in Halo is also a rigid body, when they collide, it causes the gun to spin, thus creating the Spinning Gun in Halo: Combat Evolved "glitch". Some people find extreme amusement in this; others see it simply as one of those little tidbits in the game that is interesting at first but quickly becomes something to be ignored, whereas some people just ignore it all. This can also sometimes work with health packs too but shot the edge of the pack to watch it spin.
Those who find interest in this "glitch" usually prefer to do this with an Assault Rifle, as it makes the chosen gun spin faster.
Note: Only guns that were dropped or guns from multiplayer can be used. Guns from weapon caches are not applicable.