Nighthawk Custom (FM Series) Match Grade Bushings are machined premium-grade stainless steel for maximum reliability and outstanding service-life. This bushing is designed for use with .580" O.D. barrels with the I.D. and O.D. left slightly oversize for custom gunsmith fitting to your barrel, further ensuring precise lockup and improved accuracy. Notes:This is the same part that Nighthawk Custom Gunsmiths use in the build process, designed and manufactured in-house by Nighthawk Custom. Dimensions are intentionally larger then GI Spec to ensure a precision fit when installed by a gunsmith.