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You cannot Jose. They are physically different enough that they will not fit/mate up. You will need either a 308 upper and lower or 5.56/223 upper and lower. --- (308 top/5.56 bottom) --- https://www.google.com/search?biw=1920&bih=938&tbm=isch&sa=1&ei=FllDW8PNLezm_Qam2Y3IAw&q=308+lower+vs+5.56+lower&oq=308+lower+vs+5.56+lower&gs_l=img.3...55019.61168.0.61534.11.11.0.0.0.0.62.621.11.11.0....0...1c..64.img..0.0.0....0.MxH88seKzA8#imgrc=jXKiyGNAKJ1lZM: