I have a used DPMS AP4 upper for sale that has a Daniel Defense LiteRail installed as well as a set of GG&G back up iron sights. The Daniel Defense LiteRail allows for a free floating barrel. I have been running this upper using iron sights and they have been solid but this would definitely be the ideal build for anyone looking to mount optics as well. Asking $650 even. For $750, will sell with bolt carrier group and charging handle.