This rail is designed to fit 9mm receivers, there may be fitment issues with the 40cal/10mm receivers. Specifications for Midwest Industries MP5 Optic Picatinny Top Rail: Manufacturer: Midwest Industries Color: Black Finish: Hard Coat Anodized Fabric/Material: 6061 Aluminum Gun Rail: Picatinny Gun Type: Submachine Gun (SMG) Gun Make: Heckler & Koch Gun Model: Heckler & Koch MP5 Weight: 3.3 oz Condition: New Caliber: .355 Caliber Country of Origin: USA Features of Midwest Industries MP5 Optic Picatinny Top Rail Easy no gunsmith install, all tools included Allows the user to mount an optic and a light or laser while retaining the factory handguard Patent pending rock solid design Constructed of 6061 aluminum, hard coat anodized for a lifetime of service MI Lifetime Warranty Package Contents: Midwest Industries MP5 Optic Picatinny Top Rail