You may think about buying a M4 style upper. Since you have a preban, you might as well have the lug and flash hider. The RAS or RIS should both fit a upper with lug. My first ARs had heavy barrels, but I think now that was a mistake. The M4 or Goverment A2 profile barrels are lighter and don't make the carbine so front heavy. If you want to build the trendy tactical preban carbine, the M4 with RAS, Knights vertical handguard, Aimpoint ML2, Surefire M962P is the way to go. See the pics of Wes's guns in the MSTN ads in Equipment Exchange.