Jim -
See "Ordnance Report of the West Tennessee Brigade . . ." mid-to-late November 1863. OR vol. 31, pt. 3, p. 731. This isn't as great as I recalled, but is a detailed inventory of one of Forrest's brigades. Believe you are right that the Austrian rifles (and the "Mississippis") are .54 cal, which is the ammunition Forrest is calling for.
John