Me and a buddy of mine are both working on our first upper build. We have every thing we need except the hand guard end cap. We are both using 16" barrels and mid length gas system. We are not using free float hand guards. I am using 10" quad rail and he is using a 12". Low pro gas blocks so our rails will go past the gas block. I'm just trying to find out what kind of end cap we need? We already have delta rings.