Our take: The Sapien Arms performance 6.03mm series of tightbore barrels are the newest tightbores out on the market. They feature a heavy metal Ni (nickel) coating for high lubricity and performance, and increase performance (power) an average of 15% with installation.
The super slick Heavy Metal Nickel Finish strengthens and protects the barrel, while reducing friction (conserves velocity & back spin). This 363mm barrel fits most M4 / M4A1 Carbine / SG551 metal gearbox AEGs.
Precision manufactured in the USA, each barrel is HAND TESTED for tolerance and to ensure quality.
Note: 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.