Harris Engineering Number 4 Universal Adapters. OpticsPlanet Inc is an Authorized US Distributor for Harris Engineering Bipods. Requires two inches of uncluttered barrel for mounting. Generally fits over solid ribs. Fits barrels from .550 inches to .812 inches in diameter. Requires six inches or longer barrel. Not recommended for shoulder guns. Best for single shot pistols. Features hinged mounting bracket that allows legs to fold backward. Package Contents: Harris Engineering Number Universal Adapters 4 We offer the complete line of Harris Engineering Bipods. For our complete selection of products by Harris Engineering on sale please visit our Harris Engineering page. For more Bipod Accesories please visit our Bipod Accesories store section. Harris Engineering Number 4 Universal Adapters