CMC Triggers AR-15 5.56/.223 Caliber Bolt Carrier Group Mil-Spec Nickel Boron Nickel Boron Small. This one is mil-spec dimensioned for 5.56 and .223 and it's finished in nickel boron. That coating is slick enough that carbon wipes off with a rag instead of a scraper and it cuts friction inside the receiver too. Many shooters run these with very little lube. If you've ever spent an evening scrubbing a BCG you'll appreciate the difference. Specifications for CMC Triggers AR-15/10 5.56/.223 Caliber Bolt Carrier Group: Manufacturer: CMC Triggers Color: Black Fabric/Material: Steel Gun Make: AR Platform Gun Model: AR-15 Package Contents: CMC Triggers Bolt Carrier Group 5.56 NATO/223 Remington Fits AR-15 Black What firearm models does the manufacturer say CMC Triggers AR-15/10 5.56/.223 Caliber Bolt Carrier Group is compatible with? The manufacturer says that CMC Triggers AR-15/10 5.56/.223 Caliber Bolt Carrier Group is compatible with AR-15CMC Triggers CMC-AR15-BCG-556-NIB: AR-15 5.56/.223 Caliber Bolt Carrier Group Mil-Spec Nickel Boron Nickel Boron Small