Our take: This outer barrel set is constructed out of highly durable steel, ensuring long lasting durability. This set comes with an M16 length outer barrel and 509mm tight bore inner barrel that measures 6.03mm in internal diameter, providing maximum accuracy and increased FPS. Because these barrels are precision crafted, only high quality, precision grade BBs should be used with this inner barrel. Professional installation is recommended for anyone unfamiliar with installing a new inner barrel. The performance finish strengthens and protects the included inner barrel, while reducing friction (conserves velocity & back spin). This simple upgrade set will result in improved accuracy and is perfect for M16 AEGs or replacing old multi-piece outer barrels. Note: This outer barrel set is ready to use right out of the box, though professional installation is recommended/may be required. Please use only ultra high-quality precision seamless BBs in your gun after installing this barrel, as inferior grade BBs (i.e. BBs from sporting goods stores) WILL jam inside this barrel. Specifications: Inner Barrel Diameter: 6.03mm Inner Barrel Length: 509mm Outer Barrel Type: M16 Barrel Construction: Aluminum AEG Compatibility: M16 Package includes: Outer barrel, tight bore inner barrel