The unlicensed Novato teen was stopped by a police traffic officer around 9:40 a.m. in the 1700 block of Novato Boulevard for playing music too loudly in the Dodge Durango, Cpl. Nick Frey said.
The teen was alone in the SUV and the officer spotted the AT4 rocket launcher in the backseat, Frey said. The weapon was unloaded.
The single-shot, recoilless weapon was developed in the early 1980s and is made in Sweden and the United States.
Police called the bomb squad from the University of California at Berkeley Police Department, which took possession of the rocket launcher and turned it over to a local military institution, Frey said.
The teen was booked into juvenile hall, Frey said. The boy's brother, a former member of the military, also responded to the scene Frey said.
Police are investigating how the teen and his brother came to be in possession of the rocket launcher.
The incident closed Novato Boulevard near Seventh Avenue until around 1 p.m., Frey said.
Scott Bauer, deputy director of Marin County Library Services, said 50 people, including eight to 10 staff members, were evacuated from the library at 1720 Novato Boulevard during the incident.