Agreed, and welcome to our Community. I hope you like it here.
For me, if Iām going to use a rifle, I want the ballistic advantage (significantly better chances of immediately stopping the attacker) of using a rifle. 9mm in any length barrel does not make rifle ballistics (and is still lethal through the interior walls)
For me it would be 00buck in a Remington 870 tactical. A friend of mine who worked for the IRS In security. Said I should have #6 or #8s sounds reasonable make a lot of holes you canāt stop up. Hope I never have to but if it happens it happens.
@Michael1665 Welcome to our community, we are glad to have you here. 
Mossberg has introduced the 590S. It is a military grade 12 gauge pump that will reliable handle 1.75" to 3" shells. 20" barrel with sights. It will hold 13 1.75" shorty shells. Aguilla and Federal are making them. The Aguila buckshot load is 7 #4 buck and 4 #1 buck in each shell. The Federal is 15 #4 buck in each shell. Slugs available in both. Recoil is much reduced with the short shells.

Well, taking a hostage-saving head shot on the bad guy is kinda hard to do with a shotgun, so for that reason I prefer an AR with a red dot for home defense. Of course, firing off more than one round with an AR is considerably easier than having to manually pump a shotgun to get another round loaded. Especially with certain shotguns that you really have to rack back hard and fast to get consistent reloading. That could be solved with a semi-automatic shotgun, but even still. If Iām in a situation where I need to fire a gun, Iād rather have the extra ammo and not need it than need it and not have it.
On a side note, itās entirely possible to mount a light on shotguns. Youāll just have to buy a mounting kit that can attach to the barrel.
Not a prolific poster here but the topic fascinates me along with the responses and logic that goes with our choices.
For reasons I will not bother to explain my wife sleeps in a different room in the house than me. She knows that if entry is made that I will be fighting with a handgun. If I do not prevail she will be at the ready behind her bed with the 12 gauge aimed at the bedroom door opening.
Without question at room distance with 00, this is a fight stopper.
We are empty nesters. A full load into a torso dumps enough energy that any pass through will be of minimal risk of penetrating what lies beyond. Yes it will be loud as would any unsuppressed firearm indoors.
Our plan for our situation and the logic and reasoning behind it.
And yes she is very handy with the old 12 gauge.
Personal choice. I like having the pump with itās very specific sound as a potential deterrence. However, I also have the AR for volume of rounds in case they are needed to stop a threat. That being said, I got the best of both worlds, AR 12. The bolt locking into battery has a very specific sound, and the 20rd drum provides the volume. I keep #6 as to try and avoid over penetration. But in a closer environment such as a home, thatās hard to avoid.
Itās also nice for my wife since the AR 12 is light enough for her to handle effectively as with the AR 15.
Practice your drills, know your plan and hopefully never have to execute that plan.
Number 6 birdshot?
Or six buckshot cartridges?
#4 buck is the smallest buckshot size. Shot sizes are numerical and progress from #1 to letter designations.
All (3), in multiple iterations, are available. It would all boil down to which one I can access the quickest.
My pistol is the closest option, but on the topic of the question asked here is my 12ga with PDX1 loaded.
I never realed there was a 0000 buck. How many in a 12 gauge 2 3/4 shell?
Me either, made as a specialty round. 
12 Gauge 2-3/4" UltraBuck Magnum 0000 Buckshot - 10 Rounds - OUT OF STOCK (wolfhillammo.com)
I have some 3" 410 loads that are 4 .40" balls. Coming single file out of a .410" cylinder bore the pattern is very tight. At 7 yards it is one ragged hole.
Thatās interesting. Iād love to see them shoot that at a home defense distance, 10 yards max. That is probably one huge hole.
Two pellets of 0000 buckshot side by side are right at or slightly over the bore diameter of most 12 gauge barrels, depending on whoās pellets you use and the exact diameter. Notice thereās no shot cup in the images Bruce linked. If the pellets are too large, they can get dented or scatter like breaking the balls on a pool table. Itās generally a better diameter for 10 gauge.
I load 4 000 buck in a 2.5" 410 shell using a shot cup. Less deformation and tighter patterns. I have loaded 3 .375 balls in the 2.5" shell. I use a paper tube to shim them since they donāt fit in the shot cup. I can get 4 in a 3" shell. For my single shot 410 I make light shot loads in 45 Colt cases for pest control or snakes. I have 45 Colt dies. Easy and cheap.
 
  
 
  