I think you'll find that the American rejection of the German protest about the illegality of Shotguns, under Article 23(e) of the Annex to the Hague Convention IV Respecting the Laws and Customs of War on Land of 18 October 1907 had more to do with politics (and diplomacy) than it did with the actual legality of the matter. The article concerned stated (emphasis added) that:
http://avalon.law.yale.edu/20th_century/hague04.asp#art23 said:
Art. 23.
In addition to the prohibitions provided by special Conventions, it is especially forbidden -
To employ poison or poisoned weapons;
To kill or wound treacherously individuals belonging to the hostile nation or army;
To kill or wound an enemy who, having laid down his arms, or having no longer means of defence, has surrendered at discretion;
To declare that no quarter will be given;
To employ arms, projectiles, or material calculated to cause unnecessary suffering;
To make improper use of a flag of truce, of the national flag or of the military insignia and uniform of the enemy, as well as the distinctive badges of the Geneva Convention;
To destroy or seize the enemy's property, unless such destruction or seizure be imperatively demanded by the necessities of war;
To declare abolished, suspended, or inadmissible in a court of law the rights and actions of the nationals of the hostile party. A belligerent is likewise forbidden to compel the nationals of the hostile party to take part in the operations of war directed against their own country, even if they were in the belligerent's service before the commencement of the war.
The German protest, it should be noted referred specifically to "sawned off" weapons. As to whether or not such weapons caused, "To employ arms, projectiles, or material calculated to cause unnecessary suffering" could be debatable however, it was never taken before an international court of justice for judgement.
The Germans put their view. The US it's. That does not mean it has been judged either way by an impartial body. Both sides sought to further their viewpoint at the expense of the other. Personally, I suspect that the American viewpoint was a case of sophistry. If one reads the claim that,
http://www.tacticalshotgun.ca/content_sub/shotguns/da-pam_27-50-299.html said:
The only instances even where a shotgun projectile causes more injury to any one enemy soldier than would a hit by a rifle bullet are instances where the enemy soldier has approached so close to the shooter that he is struck by more than one of the nine . . . [No. 00 buckshot projectiles] contained in the cartridge.
It is put in such a way as to claim that the victims of these shotguns would not normally be close to the firer when, as they were utilised for trench clearing when ranges were short enough to ensure that the target was likely to be close enough to indeed be struck by "more than one of the nine...contained in the cartridge". This is stretching credibility IMO.
What is interesting is that,
http://www.tacticalshotgun.ca/content_sub/shotguns/da-pam_27-50-299.html said:
In August 1992, the Government of Germany issued a new law of war manual.16 Paragraph 407 of the manual states: "It is prohibited to use bullets which expand or flatten easily in the human body (e.g., dum-dum bullets) (Hague Decl 1899). This also applies to the use of shotguns, since shot causes similar suffering unjustified from the military point of view. . . ." The issue of whether shotgun buckshot violates the prohibition contained in the Hague Declaration Concerning Expanding Bullets of 29 July 1899 is addressed later in this article. Since the German manual's objection to the shotgun relies upon the 1899 Hague Declaration Concerning Expanding Bullets, it can be assumed that the Government of Germany no longer regards the combat use of shotguns as a violation of the general prohibition of weapons causing superfluous injury, contained in Article 23(e) of the Annex to Hague Convention IV of 18 October 1907, as previously asserted in its diplomatic note of 23 September 1918.
Yet, the US study where this statement was made conveniently ignores the fact that the German Government does regard the use of Shotguns still illegal, merely under a different treaty and article! :
It would be interesting if German troops serving in Afghanistan witnessed American troops using shotguns, whilst under say, German command as part of the NATO force there. The German commander would be duty bound to prosecute.