The "Butler Gun" was a 1.25" gun, mounted on the aft of the clipper Morning Light. Sounds to me like it was basically a typical rail gun.

[Source for caliber: "From Cotton Bales to Black Gold; A History of the Pioneer Wiess Families of Southeastern Texas" By W. T. Block. Originally published in "Texas Gulf Historical and Biographical Record", VIII #1 (Nov. 1972), 40-61.]

While I have never otherwise heard of the term, it does seem to be an official term (rather than just a Confederate slang term), as the Butler Gun is listed as armament on the Morning Light in the "Dictionary of American Naval Fighting Ships" by the Department of the Navy's Naval Historical Center, as follows:

Morning Light

(Ship: t. 937; l. 172'; b. 34'3"; dr. 19'; dph. 24'; a. 8 32-pdr., 1 rifled Butler gun)

Morning Light, a wooden sailing ship, was built at Kensington, near Philadelphia, Pa., in 1853 and launched on 15 August 1853. She was purchased by the U.S. Navy on 2 September 1861 and commissioned on 21 November of that year. Assigned to the West Gulf Blockading Squadron she was captured off Sabine Pass, Tex. on 21 January 1863 by two cottonclad Confederate steamers, Uncle Ben and Josiah A. Bell. Morning Light was under Confederate control for 2 days before she was burned on 23 January off Sabine Pass.