I may have posted this earlier but I'll do it a hundred times. The Masterkey is one bad ass lil bitch. You've got .223 remington (5.56 NATO) in the M-4 or AR-15 upper portion and you've got the diversity of a Remington 870 or Mossberg 500 on the lower portion. Shotgun rounds come in so many diverse types you'd go batty naming them all. If I could have one weapon it would be this one. With the Picaninny Rail systems available today you could put one of these together in about an hour by yourself.