Optics Ready Carbine series for shooters who wish to add various optical holograph, red dot or scope sights. Chambered in .223/5.56, with magazine capacity of 30 rounds or .308 with 20-round magazine. Gas piston system taps gas from barrel much like AK and FAL designs. Provides a cleaner operation with less recoil. Detented plug in gas block allows for easy cleaning. Barrel length 16 in.; weight 6.6 lbs. (.223) to 7.75 lbs. (.308). NOTE: Add $200 for .308 chambering. Price is for model number M4 5.56/16 in. 90391.