I'd have to vote for the shotgun as well. Preferably with a short (18 inch) barrel. 20 gauge delivers more than enough lead to put down a human at indoor ranges, so "more power" is not a good reason to go to 12 if that's all this thing will be doing. #3 and #4 buckshot will do the job, and do not present the same problems as #00, each pellet of which has the same energy as a light caliber bullet and will readily go through a couple walls. At 5 yards or less, "retained energy" is not something you need to worry about with the lighter buckshot.

Be aware that with a shotgun, a hit to the torso is almost certainly fatal. No firearm should be discharged as a warning or to "just wound" your target, but consider that the majority of people shot with handguns survive thanks to quick hospitalization. Where that figures into your decisions is up to you.