The car was stopped by officers at the Shell garage in A6 Preston Road around 9am.
Police said the car was stopped and searched after it was reported driving erratically earlier that morning.
Two men, both from Manchester, have been arrested on suspicion of possession of a firearm.
A police spokesman said: "At around 9am today (Wednesday, October 23) officers stopped and searched a car at a petrol station on the A6 (Preston Road) in Chorley.
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"It followed information from a member of the public that the car had been seen being driven erratically in the same area.
"Officers recovered a firearm from the car.
"Two men from Manchester were arrested at the scene.
"One man, aged 19, was arrested on suspicion of drink-driving and possession of a firearm.
"The second man, aged 31, was arrested on suspicion of possession of a firearm.
"Both remain in police custody."