This is a rather amusing glitch if you have absolutely nothing to do. Nontheless, it's still a glitch to watch out for.
In Halo: CE, make sure that you have an automatic gun with full bullets. (Note: NEVER use explosive weapons!!) Next, find any gun you'd like to demonstrate this with. Make sure you don't pick it up. Shoot it with all your clips, or until you're satisfied.
The chosen gun starts spinning around. A lot. This has its full speed and effect if you use an Assault Rifle.
(Note: Only guns that were dropped or guns from multiplayer can be used. Guns from weapon caches are not appliable.