-Precision CNC machined aluminum construction -Double O-rings for vastly improved airseal -Exact length laser engraved onto nozzle for easy visual identification while tuning -Improved durability over plastic nozzles Standard label for nozzles are by gun and version, MaxxModel measures by millimeter so technicians can tune the depth the nozzle travels, essential for BB groupings, rate of fire, and speed; depth of the nozzle travel is between .1mm to .5mm or more depending on the brand. 21.00mm is suitable for most MP5, MP5-K, PDW, G3-A3, M4, and M16's $4 USPS First Class shipping Check out my other listings!