I'm getting ready to order a new 16" upper, seriously considering the Bushy, pre-ban ARMS SIR system. I'm assuming the rails are mil spec and my ACOG, TA01NSN will fit with no problem? Also, the barrel, is this a light weight or heavy barrel? What about milling the front sight and using the flip up BUIS, or will the ACOG mount fine behind the front sight on this system? Will there be room behind ther ACOG for a flup up BUIS rear sight?