The MadBull Ultimate hop-up unit is a really innovative design for the M4/M16 series AEGs. It allows easy and precise hop-up adjustment do to the larger diameter wheel which has been relocated. The traditional style hop-up units with the small adjustment wheel on the side are usually either too hard to fine tune and rotate smoothly or too loose and vibrate loose and back off from your idea hop-up setting. This can be a real annoyance during game play. Ultimate hop-up units are fully compatible with the MadBull LED Tracer unit (optional) attachment which allows for a completely hidden tracer solution. Negating the need muzzle mounted tracer units or tracer magazines. 3 Functions in 1: (1) Tracer unit: You can attach a MadBull LED tracer unit on this Hopup unit. This is an optional accessory and will be available soon. (2) Chamber lock: With this new hop up design you will not have to worry about losing and dropping several bbs with a magazine change. This new design provides you with a way to keep bbs in your AEG and ready to use and not on the floor and even better it stabilizes the top of the magazine to ensure a secure and flawless feed of bbs into the AEG. You can take out the oring if you don't like this function. This function is designed by Brian Holt. (3) Easy adjust Hopup: Compared to traditional Hopup, our Hopup can be adjust easily. Includes MadBull Blue 60° hop-up bucking ***Inner Barrel Not Included*** How to install the hop-up bucking: Apply some silicone oil inside and outside the bucking when installing onto the barrel and into hop up chamber. Tech Note: Hop-Up Buckings are wear items and should be changed as often as feasible. And should be replaced when upgrading or anytime the inner barrel is removed from the hop up unit. Small pin holes and tears can occur when removing and reinstating used buckings. Reference: