The V2 Fusion Engine will come with Bolt Carrier Guide Bushing attached to the top of the front cylinder section. It is mainly used to prevent the mock bolt carrier from impacting the hop-up adjustment wheel on certain models of M4's.

This is an OEM replacement in the event yours gets taken by the workbench gremlins. The 8-32 screw which secures the bushing to the cylinder comes as part of the Fusion Engine Screw Set.