It's a real-life Grunt Birthday Party, with a real gun, real bullets, a real exploding Grunt skull and cheering children in the background.
The Grunt Birthday Party is triggered by a special human skull hidden in the Crow's Nest level of Halo 3. Various skulls are hidden throughout the game, adding humorous or challenging gameplay twists when activated; the Grunt Birthday Party skull causes the skulls of Grunts to explode with a loud popping noise when they're shot in the head, showering colorful, damaging confetti everywhere while the sound of cheering children plays in the background. It's undeniably bizarre, but not nearly as bizarre as making it happen in real life.
Bizarre and awesome, that is. A Halo fan with guns, explosives, some household miscellanea and time on his hands built a blue Grunt model with a head stuffed full of confetti, then took it out back and started popping shots at it. A high-speed camera captured everything in delicious slow-motion and the inevitable YouTube video even includes a quick "making-of" segment for those who want to try this at home.
"Don't try this at home," Bungie warned.