So I purchased from Aero Precision their .308 carbine buffer system (Buffer tube, Buffer, Spring) My question is. How do I go about choosing a gas system for the .308 carbine length buffer system when it comes time to purchase a barrel? I have not seen any carbine length 16 inch barrels anywhere online, so would a 18 inch mid length gas system work with the buffer system I have? Please help because my build is at a stand still now.