I don’t think Gun Jesus is a member here.
In the AR15 design the bolt is suspended on the spring and buffer which rides inside the receiver. The receiver halves are internally tube shaped between both upper and lower. This is why the bolt carrier is cylindrical in shape . Early AR15 used a guide rod in the design this was replaced later with the buffer weights which was done to reduce issues of bolt bounce caused by the Army using Ball propellent in the 60s. No external rails.I still don't understand how the bolt carrier slide inside the receiver. The receiver is hinged right at the "tube" that suppose to guide the bolt carrier in AR-15, does the bolt carrier simply hanging by bolt head connecting to barrel extension if the receiver is opened? or is there some kind of guide rail inside the receiver?