The upper receiver (to which the barrel with its chamber are attached) and the lower receiver are entirely separate parts in AR-15 style rifles. If the lower receiver has either .223 or 5.56 stamped on it, it does not guarantee the upper assembly is rated for the same caliber, because the upper and the lower receiver in the same rifle can, and frequently do, come from different manufacturers – particularly with rifles sold to civilians or second-hand rifles that have been repaired with spare parts. Since all parts are interchangeable, a shooter "must" take great caution to check for markings of 5.56×45mm on the barrel before attempting to fire 5.56×45mm NATO ammunition out of it.