Shotgun is not included in the list because it is not a comba[]t weapon, but no doubt a good self defense weapon.
The shotgun is a highly effective tool. The only reason militaries don't typically use them is they have been prohibited by international treaty.
Also, figure this - an AR15/M16 holds 30 rounds (military configuration). An 8-shot 12 gauge with #00 buckshot can send down range: 8 rounds x 9 pellets = 72 projectiles.