EMG x Retro Arms CNC Machined Aluminum Rotary Hop-Up Unit for M4/M16 Series Airsoft AEGs (Color: Black)

"Keep in mind these are designed for the Retro Arms outer barrel lug. Though it can work for other TM spec M4 outer barrels it works best inside a Retro Arms outer barrel attached to a Retro Arms upper as the specs are slightly different than most TM spec M4s. I dont really understand why EMG did the endorsement since almost no one runs full retro arms M4s because the body parts are Very expensive and sort of proprietary. Retro arms has High quality parts for sure but look out for their beautiful outer barrels and hop chambers not working well without each other.