A joint resolution adopted by Congress August 5, 1876, declares that--
Whereas it is ascertained that the hostile Indians of the Northwest are largely equipped with arms which require special metallic cartridges, and that such special ammunition is in large part supplied to such hostile Indians, directly or indirectly, through traders and others in the Indian country: Therefore,
Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, That the President of the United States is hereby authorized and requested to take such measures as in his judgment may be necessary to prevent such special metallic ammunition being conveyed to such hostile Indians, and is further authorized to declare the same contraband of war in such district of country as he may designate during the continuance of hostilities.
To carry into effect the above-cited resolution, the sale of fixed ammunition or metallic cartridges by any trader or other person in any district of the Indian country occupied by hostile Indians, or over which they roam, is hereby prohibited; and all such ammunition or cartridges introduced into said country by traders or other persons, and that are liable in any way or manner, directly or indirectly, to be received by such hostile Indians, shall be deemed contraband of war, seized by any military officer and confiscated; and the district of country to which this prohibition shall apply during the continuance of hostilities is hereby designated as that which embraces all Indian country, or country occupied by Indians or subject to their visits, lying within the Territories of Montana, Dakota, and Wyoming and the States of Nebraska and Colorado.
U. S. GRANT.
Ulysses S. Grant, Executive Order Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/203553