I am new to ARs and some things do not click. First, on gas systems, what does mid length and carbine length mean? How do you tell the difference between the two? How do you identify a gas piston rifle? Then there is barrel length, this part really does not compute, why does everyone order a barrel that is 11.5 inches long then add a fake suppressor that is 5 inches long to bring the OAL to a legal 16 inches or so? Why dont they get a 16 inch barrel and enjoy the extra velocity? I have a AR-15 from Armalite that has a regular stock and a folding stock with 2 different buffer springs and tubes. Are they interchangeable?