The .303 cases formed to .30-40 wouldn't chamber in most .303 rifles, but would be harmless if they did. As long as the bullet was flat-based and not unduly heavily jacketed, it would probably give reasonable accuracy. The OP may not even have a .303 to encounter this imaginary danger in, but if he did, the answer lies in a large felt-tip to write ".30-40 Krag" on the box. How many of us identify our cartridges by reading the headstamp before shooting?
It even used to be fairly frequent practice in Canada to shoot .303 cartridges in rifles chambered for the Krag, although they may been the stronger 1895 Winchester. It isn't to be recommended, especially in the Krag, but we don't seem to hear of mishaps. Making 8x57 from .30-06, however, is recommended, by John J. Donnelly in his "Handloader's Manual of Cartridge Conversions". Still, it is only the standard work on the subject. What did he know?