What is the world's most powerful shotgun?
Powerful in which way? Artillery pieces are capable of firing
flechette rounds, essentially making them oversized shotguns.
As for the shotgun as defined as an individual weapon, the
largest bore I'm aware of was the six gauge shotgun. Not a very
practical battlefield weapon, however. Militaries typically use a
.12 gauge shotgun, chambered for either 2-3/4 or 3" shotshells.
Western nations lean more towards low brass munitions, while former
Bloc countries and their client states lean more towards high brass
shotgun shells.
Military shotguns aren't the most powerful - .12 gauge is pretty
run-of-the-mill. However, some models are capable of pushing out an
impressive amount of firepower, to include fully automatic fire,
such as the Atchisson AA-12 or Pancor Jackhammer.