My go-to house gun is my 14.5" AR15. It's a BCM BFH mid length upper receiver on a spikes lower I had laying around, and has Daniel Defense lower parts with a Geissele Super Tricon trigger. It currently wears a Battlecomp 1.5, a Surefire Scout light with the 600 lumen TNVC head, VCAS sling, and a T-1. Ammo is usually 55gr fmj, but I'm looking to go toward a similar weight duty round that does not penetrate very far, especially in building materials.
Reasons for going 14.5" - No tax stamp needed, so it's much easier to train with when going out of state (most local training occurs in WV instead of VA), and it's short enough to be handy in tight spaces.
Reasons for ammo - It's what I currently have the ability to afford with, and shoot enough of to verify function and accuracy. I'm working on an ammo budget to allow duty type loads such as the Gold Dot load, and I'd like to have at least a few magazines worth of ammo, plus several hundred rounds to function and accuracy test with.
I live in a townhouse in a relatively tight suburban area. I have an end unit, but other units are located in all cardinal directions, so stray rounds are bound to find something I'd rather not have them hit. Limiting overpenetration is a huge concern. I also have a family and neighbors who all have families with small children. It's definitely something I consider.
I keep a couple spare mags in my dresser, as well as a couple at my computer in the basement, which are the two places I'm bound to be at home when incidents are likely. During the day, I'm nearly always carrying a handgun, even at home. I also have a few spare pistol and rifle mags in my get-home bag in my car.
I'm considering building a short barrel, suppressed 300 blackout and maybe using that for home defense, but, again, I'm worried about overpenetration, especially with a heavier, slower round. However, the reduced sound signature (more of a consideration of my family than just of myself) inside the house would be quite welcome should I ever be unfortunate enough to need it.