If you are close enough and in the right place you could probably kill a blue whale with a 12 gauge but in all honesty it is hardly the right weapon, nor for a whole host of quarry species. Doesn't your prey deserve a little more repect than I wonder what a bird gun could drop
? You might get it right on occasions but you sure as hell will also get it wrong and all you will do is injure. Whatever you've aimed at is only going to die once at least make it as quick and painless as you can.
There is a huge range of firearms dedicated to whatever your interest is, it's all part of the fun learning about them and becoming proficient with them before using them on a live target.
Shotguns have there place but on big boy's targets you need big boys weapons.