A 41-year-old constable, who has been suspended from duty, will be questioned on suspicion of the theft of shotguns.
It follows the arrest of another Pc, aged 46, over the suspected sale of guns handed to police for destruction.
The Independent Police Complaints Commission (IPCC) is supervising the force’s inquiry, thought to relate to more than 100 weapons.
The investigation began after a rifle was stolen during a burglary at a farm in County Durham.
It emerged that its previous owner handed the gun to police for destruction, but it disappeared from the force’s registration system.
The 41-year-old officer has worked in firearms licensing, said Durham Police.
The 46-year-old, who has also been suspended from duty, was questioned on suspicion of malpractice in public office, theft, forgery and deception.
He has since been bailed pending further inquiries.
Speaking earlier this month, Ch Supt Ian Macdonald said the matter first came to light in February.
All the weapons had since been traced to licensed firearm certificate holders, he added.