But the technology was not unknown in the Civil War.
"Before being sent on his mission to Richmond, and about the middle of November, 1864, Douglass was engaged with Clay and others in a plot to assassinate the President and his Cabinet. The conspirators met two or three times at Douglass residence, and the plans for the execution of their schemes were twice discussed in Mrs. Douglass presence. There was considerable difference of opinion amongst the conspirators as to the best way to effect their purpose. Each appeared to have a plan of his own. Among other ways it was proposed to use air guns, and the power of an air gun was tested in Clays presence in the rear yard of Douglass residence. One evening Clay said that it was easy enough to put Lincoln and Grant out of the way, but that no plan would answer but one that would secure the destruction of Stanton, Seward, and Johnson at the same timethat a clean sweep should be made of it." O.R. Series 2 - Vol. 8. Page 940
More military history of the air gun