Mossberg 590a1 shockwave picatinny base mount and shell holder combo 12 gauge.Keep your spare shotgun shells right where you need them with the Monstrum Side Mounted Shell Holder with Picatinny Rail Mount. Built for use with Mossberg 500 and 590 Series shotguns, the shell holder accommodates up to six 12 gauge shotgun shells. The ambidextrous design allows for installation on either side of the stock for shooters of all orientations. The two piece design is built to be mounted together as a single unit. Shell holders or rail