If .40 or .45 is your choice, and it for indoors specifically, look into something like Glaser Safety Slugs, which is a special kinda shot "shell" at extreme high velocity, hard enough to penetrate bone, but then opens up and the shot shreds the innards of the BG. High percentage (96%) of one-shot stops. If you put one into a wall, it'll likely not travel very far, and would be safer (thus the name) than a solid chunk of lead or copper. Not for long range, though, as the "slugs" start to tumble after they've travelled a few yards.
Here is a link: