A routine traffic stop in Boston on Saturday led to the arrest of a Mattapan man when, State Police said, they found an arrest warrant out for him for skipping jury duty and uncovered an illegal firearm in the rented car he was driving. A trooper pulled over Jamil J. Campbell, 39, for failing to stop at a stop sign at the Heath Square rotary on the border of Roxbury and Jamaica Plain. The trooper discovered that Campbell was not authorized to drive the rented car he was in and that there was a warrant out for Campbell for failing to attend jury duty in Boston Municipal Court, according to a statement from State Police. After arresting Campbell, the trooper searched the car and found a 9-millimeter handgun and a box of ammunition in the car, even though Campbell does not have a firearm’s license, State Police said. Campbell was charged with failure to stop at a stop sign; use of a car without authority; unlawful firearm possession without a license; carrying a firearm with ammunition; possessing ammunition without a license. He will be arraigned in Roxbury District Court on Monday.