One mini rifle, one 765 mm pistol and several rounds of ammunition were recovered from them, Bhojpur Superintendent of Police (SP) Navin Chandra Jha said.
The SP said all the nine were arrested from a place on National Highway 30 near Bajrang chowk under Dhangai police station in the district.
The operation was conducted by a police team led by Jagadishpur ASP Rajesh Kumar on the basis of a tip-off about the culprits assembling there to commit road robbery.
The SP said the gang led by one Arvind Pal used to commit crimes on national highways.