"To all the folks that are saying that the 30 US is what I've got, that's not correct. It says "30 USA" that's different. I'm wondering if this was foreign made or something like that. Does anyone's reference material say anything about it being called a 30 USA specifically?"
Roamin, please go back and read my posts. I've answered your question.
Your cartridge is a .30-40 Krag manufactured in the United States by Remington-UMC.
.30 USA was a standard headstamp used by Remington for the .30-40 Krag for many years.
Your round was made the Remington-UMC ammunition plant in Bridgeport, Connecticut.
Union Metallic Cartridge Company was established in Bridgeport in 1867 and manufactured there until 1912, when Remington took controlling interest in the company.
After 1912 and until 1970, REM-UMC headstamped ammunition was manufactured in the exact same plant in Bridgeport.
I posted at least one picture of one with the exact same headstamp.
As others have said, Cartridges of the World is a nice introductory survey, but it is not authoritative.
It is riddled with errors (most minor, some really -CENSORED--CENSORED--CENSORED-?!!!?) in all editions.
When you are dealing with old cartridges you MUST keep one very important point in mind -- headstamps are FLUID. Not every manufacturer used the same designation for the same round, and identifying headstamps can change over the years.
Over the years I have seen the following commercial headstamps on .30-40 Krag cartridges:
.30 USA
.30 US Army
.30 Army
.30-40
.30-40 Krag
.30 Krag
.30 USG
.30 US Gov.
ALL of those headstamps, and very likely more, have been used by companies over the years to identify the SAME cartridge -- the .30-40 Krag.
Simply put, you have a common, run of the mill cartridge with a common, run of the mill headstamp that Remington stamped on MILLIONS of .30-40 Krag cases over the years.