A 12-year-old boy was caught with a makeshift gun in his backpack at a Brooklyn middle school Monday morning, cops and law-enforcement sources said.

The child was found with the weapon — a BB gun converted into a .40mm pistol — at Middle School 577 Conselyea Preparatory School in Williamsburg just before 10:30 a.m., according to law enforcement.

The gun was not loaded, and no ammo was recovered.

The boy was charged with criminal possession of stolen property, police and sources said.

No injuries were reported at the school on North Fifth Street.

— Additional reporting by Anthony Blair