Frangible buck..Most town homes are cheaply built, most home defense shootings are 7 feet or less. Over penetration or rouge pellets would be a big concern...At least frangibles would have a chance to break apart of they hit drywall or plywood.
I watched lots of youtube videos on this subject. What I came away with is that at distances of 15 feet or less (that's a likely distance in a town house), #6 birdshot will blow your face off (in 12 ga.) And it will go through clothes, ribs, and a couple walls just fine. That's 12 ga not 20ga, but the results were so devestating, I'm sure 20ga would also be effective. I saw tests where they wrapped a largeroast, sandwiched between 2 racks of ribs in both multiple layers of denim and other tests where it was wrapped in boot leather. At 5 yards and less, the impact was devestating.
I don't think over penetration is a myth at all. If it makes it through just 2 layers of drywall, that's into the next room. It doesn't take a lot to penetrate that. In a TH, the shooting distance is going to be pretty close.
Check out this video, 3/4 of the way through, the guy shoots his meat sandwich with a 28ga #9 birdshot to show the LEAST powerful shotgun round he could test. Extrapolating what a 20ga. #6 or #3 would do, You be the judge but I think it's pretty effective. And those gages will make it to the next room, but probably not with a lot of energy, especially the #6. Remember, this is just 1 video, there are tons of others. Most people are testing for home defense at like 15 to 20 yards...that's way too far. Also, FBI doesn't recommend anything less than 00 buck in 12ga for defensive rounds. But I'm telling you, the videos speak for themselves.
https://youtu.be/zaR1EVybUgc[URL="S...etration concerns so I've got 00 in my mossy.
Bird shot has very low penetration.
So low chance of incapacitation.
Discussed many times here. Your choice, your life. I vote #1 Buck for MY LIFE.