So, I am gonna have my buddy (who is a FFL and has a MFG. Liscence) make me a short shotgun. I am gonna base it off of the Serbu Super Shorty, but want it a little longer to hold more shells. What should I base it off of, a Mossy 500, or Remmy 870? Also, should I go 12GA. or 20GA.? I am thinking I want it to hold 3+1 or maybe 4+1. But I want it to be short enough to be considered a pistol, so I can carry it in my truck.