I do not own an AR but recently stole a friends to inspect for a possible big bore whisper rifle build. I want to run about a .500 round in this build. It became obvious after inspection of the AR platform that they were designed for the 5.56 and the .223 only. The action will handle a bit more length but the mags are maxed out with a .223 round. I am just curious what uppers are out there for bigger rounds and if any or all of them are somehow feeding through the mag or are single shot systems? Would have to have a different mag, etc to feed through the mag I would think.