Mossberg 500/590 Series Picatinny Rail Mount is built for Mossberg 500 and 590 Series or equivalent shotguns. It allows for the mounting of Picatinny or Weaver standard scopes, red dots, magnifiers, or other optics. The rail installs via the included set screws into the Mossberg factory mounting holes. Crafter from lightweight aluminum, the rail is 5.5 inches in legnth and 1.4 ounces in weight. Please note that pre-1997 Mossberg shotguns do not come with factory mounting holes. For installation on guns without the appropriate mounting holes, we recommend consultation with an experienced gunsmith. SCREW SIZE Manufactured prior to 1997 is 6-48 Manufactured after 1997 is 8-40 (included with this product)