.950 JDJ
The 950 JDJ is one of the most powerful rifle cartridges ever developed as of 2019. It was designed by J.D. Jones of SSK Industries but is primarily a custom novelty that was originally intended for a customer who wanted an extremely powerful and large-caliber rifle that could shoot cast bullets. It is not a production cartridge, and only 3 rifles chambered in .950 JDJ have ever been produced.
The case is based on the 20mm Vulcan, trimmed down to a 70mm case length and necked up for a .95 caliber projectile.
Ballistics
There are mixed reports on the exact ballistics of the .950 JDJ cartridge.
Some cite a 3600 grain projectile at 2200 feet per second which would generate 38,700 ft lbs of energy at the muzzle. Others (such as Ian McCollum of Forgotten Weapons) mention a 2600 grain projectile at 2200 feet per second for a more "modest" 27,950 ft lbs of energy at the muzzle.
It's possible the former figure is simply misquoted.
References & Resources
An excellent video by Ian McCollum of Forgotten Weapons discussing the .950 JDJ, and the "Fat Mac", one of three rifles ever chambered in the cartridge, weighing in at a whopping 60 pounds.