Use what ""THEY"" use. The Daniel Defense MK18 Upper Receiver Group features the railed forend Daniel Defense currently provides to USSOCOM and a 10.3"" Cold Hammer Forged Barrel. The MK18 Rail Interface System (RIS II) features the Bolt-Up System has been rigorously tested prior to fielding by USSOCOM. The 10.3"" barrel is the same length in use for the MK18 upper most commonly used for CQB operations. The new independently ambi GRIP-N-RIP Charging Handle accommodates left- and right-handed shooters and is engineered for suppressed fire"" with anti-gas features that redirect gas flow upward and away from the face and operator. All Daniel Defense complete upper receiver groups come standard with an M16 profile bolt carrier group. NFA DISCLAIMER: Once this part is assembled into a complete firearm"" it becomes a regulated item by the National Firearms Act (NFA). Please check with your local and state municipalities for any further regulatory requirements. It is the buyer's responsibility to make sure they comply with all NFA rules and applicable laws.