The Merced Police Department conducted a traffic stop for a stop sign violation just before 4:00 am.
The driver, 22-year-old Eric Hernandez, was arrested on suspicion of DUI.
During a search of the car, officers found a semi-automatic firearm between the passenger seat and the center console.
The passenger, 25-year-old Miguel Andrade, was also arrested.
Both were booked in the Merced County Jail.