Thus, for example, ATF has administratively ruled that .50 caliber Remington Army centerfire cartridges designed for use in antique Remington Rolling Block pistols is modern ammunition because it is shootable in currently made Sharps replica rifles chambered for the .50-70 U.S. Military rifle cartridge. Such rulings have not been tested in court, however, and again, such custom made black-powder cartridges are obviously designed for use in antiques only. Modern revolvers made since 1899 chambering these do not use obsolete black powder, hence this writer concludes that they do not constitute ammunition under the federal law definition. Take standard 30-40 Krag centerfire ammunition, for example. Currently manufactured by the big ammo makers, it is designed for use in all 30-40 Krag rifles, not just those that are antiques, but also those made between 1899 and 1903.
Obsolete rounds are a different story.
There are hundreds of different ones. This author is currently seeking ATF classification as antique ammunition of about 100 of them. Just to get ATF to concede in writing to these took a lawsuit as to the .43 Egyptian-Remington and the threat of a lawsuit as to the other two. It seems doubtful that a federal criminal trial jury would. Although not mentioned earlier to avoid confusing the reader, an essential proof element of the federal crime of unlawful possession of firearms or ammunition by prohibited persons is the interstate commerce requirement.