Thanks to all for the input! Like I said, I'm sorta clueless here but trying to learn. What I think I know, so far: My AP4 rifle has a Carbine Length buffer, spring, and tube. This means I can use the ARFX stock in carbine length using the hardware already in the rifle. If I want the rifle length stock, I need the correct (A2 length??) buffer, spring, and extension tube. Sound right?