What is the least powerful hunting gun?

Answer 1

The least powerful gun is the 22 caliber rifle.Improving answer:

A ".22 short" or ".22 long" cartridge is less powerful than a ".22 long rifle" round. And for the ".22 long rifle" cartridge, if you fire it in a handgun it will be less powerful due to the shorter barrel not being able to burn all the gunpowder before the bullet exits the barrel. So handguns are generally weaker than rifles in the same caliber using the same ammo.

And I would point out that a shotgun with small birdshot loads has a much shorter range than even a .22 rifle or pistol. .22's are dangerous within 1.5 miles or about 2500 meters. A shotgun that fires a shotshell using small birdshot pellets (#9 to #7, let's say) might only be able to throw those pellets a maximum of 200 meters or so.