These Matrix-Arms free float rails are exactly what you need to complete your project rifle. The Foxtrot style KeyMod Handguard is a lightweight free floating handguard featuring a continuous Mil-Spec 1913 Picatinny rail. The rails themselves come in 6.5in, 9in and 11.25in to meet all of your requirements. Constructed out of 6061 T6 Aluminum and hard coat anodized for superior finish and strength. The light weight design can support lights, sights, lasers, and all other accessories you need. Unlike other after market rail systems these line up perfectly with your upper receiver. The quality and price come together for the best combination of value and style. The inside diameter is 1.3" to fit most gas blocks and muzzle devices..