Ultimate Arms Gear Picatinny Aluminum with Storage Battery Compartment For Beretta CX4 Rifle Or 12/20 Gauge A Storage Compartment, Stores Batteries or other Small Accessories. O-Ring Sealed on Cap to Secure Inner Compartment. Water Proof O-ring for 2 pcs CR123A Batteries Storage. Works with all Weaver/Picatinny Systems. Steel Insert for Most Secure Installation. Easy Installation !