This article uses an in-universe perspective.
The NATO 5.56 PIE cartridge (also known as a Cherry PIE round) was the staple cartridge in the reclamation of North America from the Zombie Horde in World War Z. Usually fired from the Standard Infantry Rifle, the Pyrotechnically Initiated Explosive round was designed to inflict maximum damage by superheating and exploding after striking its target. Partial headshots and neckshots to zombies became killshots as the area around the wound would burn up and explode. The Cherry Pie nickname is drawn from the reddish fiery glow that sometimes emanated from the orifices of the zombie skull when the round entered the head.