black-eyed susan, n. — Green’s Dictionary of Slang

black-eyed susan n.

[? the black ‘eye’ of the barrel]

(US, Texas) a revolver.

1869

‘South-Western Sl.’ in Overland Monthly (CA) Aug. 126: Among the names of revolvers I remember the following: Meat in the Pot, Blue Lightning, Peacemaker, Mr. Speaker, Black-eyed Susan, Pill-box, My Unconverted Friend.