jm, I know you are more intelligent than this. Almost anything will do the job if delivered with precision. The problem is that in an emergency that may take place in the dark of night and with one full of adrenaling it isn't so easy to deliver a pricise shot and that is why the 12 or 20 gauge is a much more prudent tool than the .410 unless for some reason one is limited to its use.
What I have tried to explain is that given a less then precise shot, the odds of delivering a threat ending punch is more likely with the larger gauge. If you get half your .410's five pellets outside the vital area you may get 2 or three inside it, but with a 12 you have ten pellets (assuming Winchester Magnum Super X 3" 000 BS) and you are likely to get two more hits into vitals.