Let me clarify that the cases in question were a lot of experimental cases made in 1890 before there really was a .30-40 or a U.S. Krag rifle.* But History of Modern US Military Small Arms Ammunition notes that the cases were brass, "tinned inside and out to better resist corrosion" and "were to be loaded with smokeless powder." Your statement about early smokeless powders corroding cases seemed to tie in with that information, but it does not specifically say that the "corrosion" was from the powder.
*Springfield was experimenting with the .30 caliber, using modified single shot trapdoor rifles, as pictured in Frasca and Hill's The .45-70 Springfield.
Jim
*Springfield was experimenting with the .30 caliber, using modified single shot trapdoor rifles, as pictured in Frasca and Hill's The .45-70 Springfield.
Jim