2. Why 5.56/.223
.223/5.56 is one of the best HD rifle rounds when you look at effectivness vs. overpenetration. It doesn't have the tendancy to go through walls as bad.
.223/5.56 weapons also have very low recoil, so anyone in the house should be able to use it in that respect. A good AR with a collapsible stock will also allow easy and quick length adjustment for any user, and one built with a bit of attention to lightness can also be easier for all household members to use. It has a nice 30 round magazine that can be changed very quickly vs. loading a 12 gauge (at least the Saiga has a detachable box mag) as most using a 12 gauge shotgun for HD go with a tube magazine, pump action shotgun.
Everybody in my family, using my AR with a RDS, gets fast, effective hits on targets, and they do much better than with a 12 guage shotgun.
I used to be a 12 gauge shotgun is the only way to go for HD guy until I got some rounds through an AR-15.