The new M4 Advanced Colt Carbine Monolithic (ACC-M)* One-Piece Upper Receiver incorporates all the options operations forces have required. Its one-piece upper receiver provides a true free-floating barrel for increased accuracy, as well as repeatable zero retention for optics and other accessories. All variants of the ACC-M have ambidextrous fire controls, selector levers, and magazine releases. The M4 ACC-M is available in an array of configurations, from 3-Round Burst to Automatic modes of fire, as well as with fixed or folding front sights. These options, combined with the Colt Monolithic Rail System, provide unmatched zero retention and repeatability not found with other separate forward (handguard) rail mounted systems. These attributes make the M4 ACC-M one of the most versatile and the truest operational weapon system for today’s battlefields.