From Small Arms Defense Journal:

In order to understand the .408 Cheytac anti-materiel rifles, it’s important  to start with the cartridges. The parent cartridge was a .505 Gibbs; developed in the early 1900s (1911?) as a dangerous game cartridge originally firing a 525 grain bullet at 2,300 feet per second for 6,180 foot-pounds of muzzle energy.  Very effective on anything that walked – on both ends.