i had an idea on a 5.56 SCAR SSR. the normal route is with a pws rail extension. but i think the PWS extensions are ugly. so i wanted to invest in a Mk20 upper receiver and mate it to a Mk16 lower receiver. i dont have either rifles on hand to determine the difference between .308 and .223 style lowers. would anybody happen to know if they will mate up to the same upper reciever? and if not can anyone get me measurements on both lowers so i can start designing an adapter? thanks in advance guys.