The AR-15 Bore Guide is constructed of precision CNC-machined aluminum with a durable anodized finish. It fits securely in any AR-15 upper and can be retained in place by closing the dust cover on the upper which engages a groove in the guide. The bore guide also has chemical-resistant O-rings for retention in uppers without dust covers. It works well with .223 and .204, extending inside the chamber to prevent solvent from leaking into the action. A solvent port is conveniently located to the rear of the guide which extends out of the upper making it easily accessible during cleaning. Inner tube diameter is .250″ (not intended for 6.8mm jag).