Smellie - The legality of shotguns has been debated back and forth. Some countries (eg the USA) have produced legal arguments that they are indeed legal but others have said no. To the best of my knowledge, nobody has ever been tried criminally for using one.
The Conventions have specific prohibitions WRT explosive rounds that are under a certain diameter or are likely to flatten on impact, but I'm unaware of any ban in them on shotguns. As to the general prohibition on things causing unnecessary suffering, I'm not sure how a man hit with 00 buck is worse off than one who's been shredded by a mortar round or had a 5.56mm mag emptied into him.