two cents
OK, here goes.
Three inch, 12 ga buckshot loads, turkey loads or varmint loads, will kick the snot out of you, especially from a short defensive shotgun. For some folks, the standard 12ga/ 2-3/4 " load is a heavy kicker too. If you foresee the good wife, your daughter or capable children using your SD shotgun, a 20 ga maybe a viable option. In those instances, a carbine or pistol caliber carbine maybe even better.
-Do not count on the racking of a shotgun to frighten or run off an intruder or adversary. Such mental gymnastics my leave you unprepared for when they do not.
-Likewise, I cannot agree with leaving a round chambered and the safety on as a ready condition for the shotgun. A "cruiser ready" storage will still provide instant readiness,but is far safer from accidental discharge. I completely believe a fully loaded firearm cannot be left unsecured or unattended. Once you get the gun in your hands, chamber a round. Racking a shotgun may reveal your position, but.....
-Announcing one's presence MAY be exactly what you should do...those stealthy footsteps in the hall could be family sneaking in late or your girlfriend, etc. Challenging an intruder (from cover and concealment) is arguably more defensible AFTER a shooting too.
- I have mixed feelings on birdshot. Really small shot, say #7-1/2 and under, especially from light claybird loads, does not do so well much beyond 7 yds/21 ft. Inside of that, it can be devastating. If I could guarantee my SD shooting would be that close, and I absolutely had to limit penetration due to adjacent dwellings like and apartment, townhouse, or motel, I would opt for birdshot. I see birdshot having a very narrow niche in the SD shotgun.