I think the first place to start is deciding if you want Glock mag or Colt mag (or HK mag if you have too much money). If that answer is easy for you, then you have a variety of choices. Also, when you say pistol length, what do you mean? I have a 4.5", 8.5", and 16" upper for my AR9. I haven't shot the 16" yet, but as cheap as these things are to build right now, especially if you don't buy a second charge handle and BCG, you're crazy not to give yourself the option to shoot the gun while your stamp paperwork clears. Even if you plan on bracing it and not filing a stamp, I'd think you'd want to consider a longer barrel like a 10.5" especially if you reload.