Also see our list of the best GTA 5 mods.
It works like this: you pull the trigger and instead of bullets, cars spray out. Car guns are useful in situations where rockets or grenades aren't enough, but they're also good for sending a message, ie, that no one should muck around with you, the car gun guy.
Vehicle Cannon Mod 1.0 is now available over at GTA5-mods.com, with all the usual caveats about potential Rockstar Club bans. A recent patch seemingly ironed out modding capabilities, but it wasn't long before people found a workaround.
If you like guns that shoot cars, you'll probably also enjoy whales falling out of the sky.