Said Coldly
Posts here indicate that just about any FactoryLoaded Buckshot shell will be more than adequate.
Remember, your foe is there armed and ready to kill you and yours!
Forget the side worries of over penetration, grouping and Birdshot!
You are in a Fire Fight to save your or your Familys lives! You want as uch punch as your Shotgun is capable of in order to put down your opponent and save your Family/Self. Why play with bird loads when deadly force is called for?
Birdshot dosen't spread at room ranges. Nedither does "00" Buck!
I load my 12 GA Pump with military 9 Ball and my 20GA SxS with 20 Ga, #3 Buckshot.
Get the best centered, fullest hit on your enemy and shoot more than once! Even though this a shotgun, a low shot count hit is possible.
End the fight if possible.
If not shoot well and get the family/self out of harms way.
If all you have is light loads, then shoot many times if possible to incapacitate/cripple your opponent. You MUST stop the agression aimed at you.