In World War Z, when the US military went on the offensive against the zombies their rifles used a derivative of the 5.56mm NATO round ("Our staple ammo was the NATO 5.56 'Cherry PIE'."). Although very similar to the .223 round voltbang referenced, there are a few small differences between .223 and 5.56 cartridges (the two are frequently used interchangeably, but they're not).