I’ve built quite a few AR uppers and never seen this. I just got a new assembled upper from Aero. When going to put on the barrel and gas system - The gas tube will not go through the hole on the upper receiver. I took it all back apart and removed the barrel and nut to make sure nothing was obstructing it. Tube doesn’t go in the receiver. I swapped to another gas tube just to rule out the tube. Still no go. It’s like the hole in the receiver is too small an opening. Anyone else ever see this? What did you do?