I am new to AR pistol so please guide me. I ordered a PSA complete "multi cal" pistol lower, and I intended to get 556 upper but now changed my mind and wanted to get a 9mm barrel. So I will need a complete barrel chambered in 9mm, 9mm mags, 9mm BCG? I beleive the lower has carbine buffer spring and buffer tube, do I need to change them? Your guidance is highly appreciated. Thank you.