OMG-Z by Laughing Jackal Ltd.

Alert! Alert! OMG-Z! The City of Redfield has been hit by an outbreak of exploding zombies! For cool and unimportant reasons these suckers literally explode when shot, showering their zombie brethren with gore and causing chain reactions of showering zombie gibs with just a single bullet! Armed with this knowledge (and bullets coated in a zombie-exploding pathogen) your elite unit is dispatched to the epicenter of the outbreak. Your mission? Succeed where the police and army failed and clean up the city! For reasons that aren't important right now, all but one of your team is killed before arriving at the outbreak's epicenter, leaving YOU to deal with thousands of zombies ALONE. Use your chain reaction skills to explore 81 waves of zombie hell across 32 environments. Earn 324 shiny medals and cold hard cash, and use it to buy 70 different upgrades all designed to MAKE ZOMBIES EXPLODE!