Caution: the Federal legislation which banned all bullets supposedly designed to penetrate soft body armor, 18 USC Sec 921(a)(17), banned all bullets made from one or a combination of Tungsten, steel, iron, brass, bronze & beryllium copper, or depleted uranium. Brass and bronze are just copper with Tin or Zinc, and sometimes other things such as Lead, Silver and other things. I have seen references to materials being "brass" or "bronze" when they are 96% copper. If the copper used was produced from scrap, I would be willing to bet there would be several percent of tramp elements.
One of the problems manufacturers have is knowing composition of something they are working on. If you pay a supplier enough, they will certify that they have brought you green cheese from the moon. If you want to lathe bullets from copper, I would as the BATFE if they feel that 18 USC 921 (a)(17) controls making projectiles from copper, and specify the source and grade of copper you are going to use (pick one you know to be low in Tin and Zinc). You can do this on-line at:
https://www.atf.gov/contact
Also, make these just for yourself, or get licensed as a manufacturer.
I have contacted BATFE on technical questions, but always done it via letter.
The above is for entertainment. Under the laws of my state, I may not represent anyone other than myself.