Thanks for the information!

I think I have this figured out. The Butler rifle was actually one of the rifled small cannon purchased by Gen. Butler. He had the luxury of being able to afford new technology if it interested him. He was one of the first to buy Gatling guns. The small mobile cannon must have seemed attractive. I believe that it was of the type known as an Ellsworth gun. The size and breach loading match. The caliber seems right. Butler definately had at least two of them in Louisiana and they saw action. Someone actually built a replica Ellsworth gun and it worked fine. The downside was that the guns were too small. They really didn't have much hitting power. An application at sea used as an anti-personnel weapon at long ranges or as a nuisance weapon might make sense. The Confederates proposed using a similar weapon (Nichols rifle) at Galveston against until boat parties. The head of the Marine Department termed them "useless".