CMMG Inc Barrel and Bolt Group Kit Medium Taper 22LR 4.5in 4140CM. 22LR. Just purchase a magazine and you are ready to go. For best performance use a round style hammer as notched hammers can cause function issues and 36 grain plated nose bullets have proven to be the best choice for function. Specifications for CMMG 22LR Mk4 Barrel Kits w/ Bolt Group: Manufacturer: CMMG Color: Black Finish: Matte Fabric/Material: 4140 Chrome Moly Caliber: .22 Long Rifle Threads per Inch: 1/2x28 Barrel Twist Rate: 1-16 Flutes: No Gun Make: AR Platform Gun Model: AR-15 Condition: New Package Contents: CMMG 22LR Mk4 Barrel Kits w/ Bolt Group What firearm models does the manufacturer say CMMG 22LR Mk4 Barrel Kits w/ Bolt Group is compatible with? The manufacturer says that CMMG 22LR Mk4 Barrel Kits w/ Bolt Group is compatible with AR-15CMMG Inc 22D5B6E: Barrel and Bolt Group Kit 4.5in. MT 4140CM 22LR