A modular storage system for an interior compartment of a safe and method for storing guns comprises standardized parts for easy construction and assembly, flexible arrangement and configurations, and use in a wide variety of safes. The interior compartment of the safe is configured to have a back wall and two side walls defining an interior width of the safe. The system comprises a plurality of shelves each having a standardized shelf width. The interior width of the safe is a whole number multiple of the standardized shelf width. A gun storage bay is provided with an exterior bay width equal to the standardized shelf width. The plurality of shelves and the gun storage bay are removably mounted on the back wall.