So I finally acquired the last parts for my first AR and finished assembling it. Problem now is that I usually can't rack the bolt, it appears to be hitting the bottom lip of the buffer tube. If I unlock the takedown pin and hold the upper and lower together by hand, then I can rack it, as it allows the upper to shift up just a hair. If I have the pin locked in, then I can sometimes rack it, with difficulty, if the castle nut on the buffer tube is just a little loose and I do a special dance. The parts in question are an Aero enhanced upper, Aero gen 2 lower, PWS EBT Mod 2 buffer tube, and a Superlative Arms piston kit bolt carrier. I'm not ruling out an error in assembly, but it's pretty simple, I don't think I screwed anything up. I'm probably going to have to take it to a gun shop nearby to figure out what the problem part is since I don't have any other parts to swap around. Is there a most likely culprit part that might be out of spec? Any common noob assembly mistakes I should check for?