Could be a Brunswick ball specially made for the Brunswick Rifle or something very simular. The Confederates are known to have ordered and used many of these rifles. They had two rifling grooves and used a special ball that had keys "belts" made into them to fit the rifling.

"Confederate Brunswicks: There is documented proof that the Confederacy used the Brunswick rifle during the Civil War. Around Dec. 1 1862, Confederate purchasing agent, Caleb Huse purchased 2,020 Brunswicks which is later confirmed in a letter by Confederate Ordnance Chief Colonel Josiah Gorgas on Febuary 3, 1863. (Firepower From Abroad, Wiley Sword, Appendix II, Page 67) The Confederates adapted a .70 Caliber Minie Ball round which was found to be more accurate than the original "belted ball" ammunition. It is believed that most of these Brunswicks were of the older first pattern with the back action lock and issued to the Trans-Mississippi Command (Firearms of Europe, 2nd Ed., Whisker, Hartzler, Yantz. P. 22-23). Confederate Brunswicks are also photographed in the latest edition of Flayderman's and in "The Fighting Men of the Civil War" by William Davis on Pgs 52-53."

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David Upton