A lengthy treatment of the subject from lawofwar.org:
On 19 September 1918, the Government of Switzerland, representing German interests in the United States, presented to the U.S. Secretary of State a cablegram received by the Swiss Foreign Office containing the following diplomatic protest by the Government of Germany:
"The German Government protests against the use of shotguns by the American Army and calls attention to the fact that according to the law of war (Kriegsrecht) every [U.S.] prisoner [of war] found to have in his possession such guns or ammunition belonging thereto forfeits his life. This protest is based upon article 23(e) of the Hague convention [sic] respecting the laws and customs of war on land. Reply by cable is required before October 1, 1918."
The German protest was precipitated in part by the capture in the Baccarat Sector (Lorraine) of France, on 21 July 1918, of a U.S. soldier from the 307th Infantry Regiment, 154th Infantry Brigade, 77th Division, AEF, who was armed with a 12-gauge Winchester Model 97 repeating trench (shot) gun, and a second, similarly-armed AEF soldier from the 6th Infantry Regiment, 10th Infantry Brigade, 5th Division, on 11 September 1918 in the Villers-en-Haye Sector. Each presumably possessed issue ammunition, which was the Winchester "Repeater" shell, containing nine No. 00 buckshot.
The German protest was forwarded by the Department of State to the War Department, which sought the advice of The Judge Advocate General of the Army. Brigadier General Samuel T. Ansell, Acting Judge Advocate General, responded by lengthy memorandum dated 26 September 1918. Addressing the German protest, General Ansell stated:
Article 23(e) simply calls for comparison between the injury or suffering caused and the necessities of warfare. It is legitimate to kill the enemy and as many of them, and as quickly, as possible . . . . It is to be condemned only when it wounds, or does not kill immediately, in such a way as to produce suffering that has no reasonable relation to the killing or placing the man out of action for an effective period.
The shotgun, although an ancient weapon, finds its class or analogy, as to purpose and effect, in many modern weapons. The dispersion of the shotgun [pellets] . . . is adapted to the necessary purpose of putting out of action more than one of the charging enemy with each shot of the gun; and in this respect it is exactly analogous to shrapnel shell discharging a multitude of small [fragments] or a machine gun discharging a spray of . . . bullets.
The diameter of the bullet is scarcely greater than that of a rifle or machine gun. The weight of it is very much less. And, in both size and weight, it is less than the . . . [fragments] of a shrapnel shell . . . . Obviously a pellet the size of a .32-caliber bullet, weighing only enough to be effective at short ranges, does not exceed the limit necessary for putting a man immediately hors de combat.
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. This, like the effect of the dispersing of . . . [fragments] from a shrapnel shell, is permissible either in behalf of greater effectiveness or as an unavoidable incident of the use of small scattering projectiles for the nec-
essary purpose of increasing [the] likelihood of killing a number of enemies.
General Ansell concluded his memorandum with the statement that "The protest is without legal merit." Acting Secretary of War Benedict Crowell endorsed General Ansell’s memorandum of law and forwarded it to the Secretary of State that same day. Secretary of State Robert Lansing provided the following reply to the Government of Germany two days later:
[T]he . . . provision of the Hague convention, cited in the protest, does not . . . forbid the use of this . . . weapon . . . . n view of the history of the shotgun as a weapon of warfare, and in view of the well-known effects of its present use, and in the light of a comparison of it with other weapons approved in warfare, the shotgun . . . cannot be the subject of legitimate or reasonable protest.
. . . .
The Government of the United States notes the threat of the German Government to execute every prisoner of war found to have in his possession shotguns or shotgun ammunition. Inasmuch as the weapon is lawful and may be rightfully used, its use will not be abandoned by the American Army . . .
f the German Government should carry out its threat in a single instance, it will be the right and duty of the . . . United States to make such reprisals as will best protect the American forces, and notice is hereby given of the intention of the . . . United States to make such reprisals.
World War I ended six weeks later, without reply by Germany to the United States response. There is no record of any subsequent capture by German forces of any U.S. soldier or marine armed with a shotgun or possessing shotgun ammunition, or of Germany carrying out its threat against the U.S. soldiers it captured earlier.
