Green’s Dictionary of Slang

(US) cheap strong bourbon.

1859		L.A. Star 23 Apr. 4: Minnie rifle, Knock-’em stiff and flaming red-eye — Such as kills ’em at the counter, Forty rods or any distance.
1860		Spirit of Democracy (Woodsfield, OH) 25 July 4/1: Those brands of whiskey known as [...] Minnie rifle, stone-fence, kill-brain, etc.
a.1865		J.H. Browne Four Years in Secessia 194: Its classic communities would turn their attention from imbibing Minié rifle whisky [DA].