Primary Arms 30mm Scope Tube Offset Mount with 1 Inch Inserts SCOPE TUBE OFFSET MOUNT: Mount the ring onto the tube section of your primary scope and attach your secondary optic to the 1913 Picatinny rail section. SIZE: For use with scopes with 30mm tubes, or use the 1-inch insert for 1-inch tubes. RUGGED AND DURABLE: Made of durable aluminum with a hard coat anodized finish.