The .950 JDJ is one of the most powerful rifle cartridges ever developed in the area of sporting rifles. It was designed by gunsmith and firearm designer J.D. Jones of SSK Industries, who it is named after, sharing his initials.
It 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.
One of these rifles is reported by AccurateShooter.com to have costed $8,000, with each round costing about $40. Value estimates on the rifles vary quite a bit. Rock Island Auction Company originally estimated the first rifle (aka "Fat Mac") at $20,000 to $35,000 USD. That particular rifle later sold for $12,650 USD at auction. It included: "... Burris 7x scope. Includes two reloading dies, approximately 23 loaded rounds, 63 empty casings, two wood loading blocks, and approximately 90 cast lead bullets" as per the auction page at Rock Island Auction Company's website.
The case is based on the 20mm Vulcan, trimmed down to a 70mm case length and necked up for a .95 caliber (24.1mm) projectile. However, some sources claim the cartridge may actually be based on the 20x110mm Hispano, a French anti-aircraft cartridge.
There are mixed reports on the exact ballistics of the .950 JDJ cartridge.
Some (AccurateShooter.com for example) 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.
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