Incendiary ammunition

Incendiary Ammunition is a special type of ammo that can be found in the Left 4 Dead sequel. After picking up this ammo, all gun shots are flammable and can catch infected on fire (with exceptions). The only evidence of this at the moment is a video that can be found here showing up at about 5:49 through 6:49. The video shows an Incendiary Ammo Pile and the effects.