I don't think you quite get it....
The inner barrel is the 6mm tube the BBs travel through. The outer barrel is the one that replicates the barrel of the real weapon. The inner barrel is inside it.
You must have both on your AEG. Unless you want it to look ridiculous. You cannot fit anything on the inner barrel.
Inner barrel:
Outer barrel:
G36k outer barrel removed to show the inner barrel(brass):
