I received a Superior Arms AR lower receiver (Model # S 15) as a gift, and paired it with an RRA 16" flat top upper receiver. However, when I mounted the upper to the lower, the rear retaining pin/takedown pin would not seat, and in fact it sticks out about .25" from the side of the lower. I took the rifle to Bullseye in Tacoma (it was not purchased from there), and they quickly identified that the retaining pin well (I am not sure of the nomenclature, but I am trying to describe the hole(s) drilled in the lower that the takedown pin fits into) is angled slightly. The hole on the left (exit) side is about a mm towards the rear when compared to the right (entrance) side, and is apparently a machining flaw. My brother, who also has a lower made by Superior Arms (the SN of which is near mine), has the same problem. I would like to know: a. Has anyone had similar problems from Superior Arms? b. Is this a common manufacturing defect with AR lowers?