Well....
My first, and fort many years, only shotgun was a Mossberg 500 in 20 gauge...And I used it for birds, deer, rabbits, whatever. And it was easier on the shoulder, so I could go out and practice more (Often 100 rounds in a day) without getting sore. Ins short, a 20 gauge with do 95% of what a 12 gauge will do, in "real life" situations...but a .410 will only do about 65% what a 12 gauge can do. My .02
My first, and fort many years, only shotgun was a Mossberg 500 in 20 gauge...And I used it for birds, deer, rabbits, whatever. And it was easier on the shoulder, so I could go out and practice more (Often 100 rounds in a day) without getting sore. Ins short, a 20 gauge with do 95% of what a 12 gauge will do, in "real life" situations...but a .410 will only do about 65% what a 12 gauge can do. My .02