The Ruger SR556CLA (CLA= Carbine, Light Adaptive) has a nice slim rail that has smooth sides and picatinny rails on top and bottom.
THE AR PLATFORM REDEFINED. The SR-556® offers Ruger's legendary rugged reliability, and comes factory standard with a host of accessories that today's shooters demand. Employing a patent-pending, chrome-plated, two-stage piston driven operating system that provides the user with a cleaner, cooler running, reliable firearm that is truly state-of-the-art.
The chrome-lined gas block and chrome-plated piston and regulator work together to provide a smooth power delivery stroke to the bolt carrier. The by-products of the gas system are vented out the bottom of the gas block, away from the bolt carrier, keeping the action clean and free from contaminants. The end result is significantly improved performance over gas driven rifles.