Submachine gun vs. Cops
The cops will call in backup if the fight goes on for too long. They will first call in a cop that fires poison gas canisters, then they will call in SWAT Cops that use a rocket launcher. Some cops will hide behind cover.
Shotgun vs. Zombies
The zombies puke maggots until they get parts blown off of them. Their behavior changes depending on if they lose their legs or their head.