CMMG Parts Kit Bolt Maintenance Mk57 5.7x28mm 57AFF37. 728) Bolt. Specifications for CMMG Mk57 5.7x28mm Bolt Maintenance Kit: Manufacturer: CMMG Cartridge: 5.7x28mm Gun Make: CMMG Package Contents: CMMG Mk57 5.7x28mm Bolt Maintenance Kit Mk57 (5.728)...