The MR556A1 Upper Receiver Kit includes a complete upper receiver/barrel assembly bolt carrier assembly buffer and buffer spring allowing most high-quality 5.56 mm AR-style rifles to be retrofitted to HK's pusher rod system. - Complete MR556 Upper Receiver/Barrel Assembly HK 16.5 inch cold-hammer forged barrel upper receiver with forward assist extended convertible charging handle HK Free Floating Rail System handguard - Complete MR556 Bolt Group Assembly - Tungsten Filled HK Proprietary Buffer - HK Proprietary Buffer (Action) Spring