Can't believe some of the stuff you hear on here...

1. Birdshot does not reliably penetrate (humans) enough to incapacitate/kill.
2. If anything, 9MM will penetrate MORE than .45 ACP (smaller bullet/higher velocity).
3. Even though it would be "stupid" to come any further if you hear a shotgun slide-racking sound, do you think your common criminal is a criminal because he could have been a brain surgeon?

For a handgun, any 9MM, 40, or 45 ACP will work (for autoloaders) as long as you are shooting quality hollowpoint ammunition.

For a shotgun, use slugs or buckshot, not birdshot or turkey loads, and try to not make it where you have to "rack the slide" unless you like to give bad guys the advantage by telling them where you are and informing them that you are armed, so they know to come in shooting...