Side mounted shell holder with picatinny rail mount, built for Mossberg 500 and 590 Series shotguns. Accommodates up to six 12 gauge shotgun shells. Ambidextrous design allows installation on either side of the stock. Two piece design mounts together as a single unit into the factory mounting holes of Mossberg 500/590 Series models with no additional gunsmithing required. Also compatible with Mossberg Shockwave, 535, 835, and 590a1 models. Pre-1997 Mossberg shotguns require 6x48TPI screws (not included).