Did you read what I wrote about penetration ?If only drywall is between you and your neighbors, get a 12ga shotgun loaded with birdshot. A .45 will penetrate through about four or five walls constructed of drywall. If the walls between you are construction blocks, then most anything except high caliber rifles should be safe.
#4 buckshot is available all over the place, in my neck of the woods, but I can see how it might be geographically restricted.I used #4 buck to dispatch foxes with as a youngster .And regular #4 to hunt ducks with. Number Four buck would likely be hard to find today , probably have to order it off the net .
Myself,
I chose #1 buck, and do have to order it ... A few more balls, and adequate performance (PENETRATION).
I also live in a considerably different situation than the OP, where I'd be recommending #4 buck, if the decision is to go with a shotgun.
ETA:
http://www.chuckhawks.com/home_defense_shotgun_ammo.htm
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