Here in Massachusetts, the law regulating guns in cars is G.L. c. 140, Sec. 131C.
According to that statute, loaded firearms inside a vehicle must be “under the direct control” of a properly licensed gun owner.
Any rifles or shotguns inside a vehicle must be unloaded and secured in a locked container.
The statutory law (G.L. c. 140, Sec. 121) defines a “locked container” as
a container that is capable of being unlocked only by means of a key, combination or similar means, including in an unoccupied motor vehicle, a locked trunk not accessible from the passenger compartment, a locked console or locked glovebox and for purposes of a common carrier in the course of the regular and ordinary transport of firearms, locked access to any area containing firearms.
Anyone convicted of violating G.L. c. 140, Sec. 131C will automatically lose his gun license for 1 year.
A conviction of a violation of this section shall be reported immediately by the court or magistrate to the licensing authority. The licensing authority shall immediately revoke the firearm identification card or license of the person convicted of a violation of this section. No new firearm identification card or license may be issued to a person convicted of a violation of this section until 1 year after the date of revocation of the firearm identification card or license.
Additionally, you could be fined up to $5,000 for violating the statute.
Finally, I should note that the statute contains exceptions that may apply to you. So be sure to read it in its entirety before you travel with your firearm.