Here is an interesting youtube video of test firing a rifle that was home built.
He also designed his own cartridge to fire it from called the .510 Deming Destroyer. The description of how it is made is in the video description:
My Homebuilt bolt action, firing it’s first round. It’s chambered in a Wildcat cartridge I designed myself, the .510 Deming Destroyer. It starts out as any WSM, WSSM, Remington Ultramag, or Short action Ultramag brass, gets cut down to .9" expanded to .508 and then turned down to .545 OD, the bullet is cast from a mold I designed and made. The only things on the rife that wern’t made by me are the Savage Bolt Head and the CETME trigger pack.