My primary load is 00 Buck Federal Low Recoil. Works great. I carry some slugs if I need penetration.
I DO NOT recommend "candy caning" the rounds in your magazine. Can you tell when your going to need buckshot or slugs? If your that clairvorent I suggest you see the fight coming and go the other way...
Some of us carry a 12 gauge during law enforcement operations, and we might fire a quick load at someone in a car, rack the slide and fire again. If the first round is buck and does not penetrate to end the fight, the second with slugs just might. If you have the time, of course, you combat load what you need based on distance and barriers. If not, you fire what you've got until the threat is neutralized, candy canes or otherwise.
I have loaded both 00-buck and slugs as the first shot out of my 870. Perhaps I am the only one, but I have no problem with the first shell being a slug, as that will handle almost any problem encountered, at any reasonable shotgun range.
If I wanted lots of little holes, I would shoot them eight times or nine times with a handgun.
I voted "other." I'm a non-LEO, just live in an apartment.
I've got a Rem. 870 with an 18 1/2in. bbl. hammer down on an empty chamber with a full tube of Federal #1 buckshot. I also still have abox of that "PD 12-D" that Federal mfr. a while back that has #2 birdshot. I notice that it is no longer marketed these days, so I haven't been keeping the shotgun in "cruiser ready" mode with that for a while.
I suspect that the PD load wasn't a big seller at the price they had it at. I don't recall exactly, but it was something like 6 or 10 rounds for more $$ than a box of 25 squirrel loads sold for.
At household ranges (unless you live in an open-plan mansion) birdshot works as well. Outside in the yard is where buck comes into its own.
I keep a few loads of BB (buffered and copperplated 2.75" mag loads) handy in case of rabid critters or a coyote in the henhouse. BB is just under #4 buck in size, but there's 75 of them per shell. If I could find them in 1.125oz field loads I'd buy them.
Light field loads can be had with #4 shot most places, without the price or liability of being named 'personal defense'.
BB pellets at range are the smallest pellets that penetrate decently.
With round projectiles, the frontal area goes up by the square of the diameter, the weight by the cube of the diameter and the coeficient of form stays constant.
Boiled down, this means that bigger pellets have higher sectional density and ballistic coeficient, therefore they retain more velocity at range, and penetrate better. Balancing this is the thinner patterns one gets with larger shot and fewer pellets per load. If the full load (or most) of the pellets hit the target then more energy is imparted.
At 20 feet there is little difference. At 30 yards, it becomes quite significant.
This is why I really don't care what kind of shell I use in the bedroom, but deer hunting I use 000 buck. Typically you'll find 000 pellets under the hide on the far side of a deer hit broadside. 000 usually patterns better than 00 also, as long as it isn't used in too tight a choke.
BB is a good load that performs well on medium-sized varmints out in the yard yet is entirely adequate inside the house. Thus it is my 'standby' load.
Were I worried about mainly needing to engage thugs at range I'd opt for buckshot or slugs unless a rifle is handy. Same for officers who might need to take out a radiator or tires.
Hope this helps explain my rationale some. Other folks' situations will be different.