West Midlands Police traffic officers stopped the vehicle overnight on Wednesday.
A search of the car found a loaded handgun with live 9mm rounds in a magazine.
A spokesman said: "We had cause to stop this vehicle following a road rage incident.
"A search of the vehicle located a loaded handgun with live 9mm rounds in the magazine.
"The driver was promptly arrested and a firearm has been removed from the streets."
West Midlands Police are yet to confirm where the incident took place.