I'm not sure why you would consider once fired instead of starting with fresh brass and forming it in your chamber?
The little Model Sevens in the RSAUM cartridge are pretty neat in my opinion. I know some will cringe at the notes about factory rifles and quality but these little rifles have worked well when I tested some for clients. And I like the cartridges but I rarely have shot any cartridge I didn't like, well, except for the .270 Win...
Once fired:
https://www.diamondkbrass.com/.300-REM-SA-ULTRA-MAG-25-ct..html
You can also run a search on Gunbroker.