The .950 JDJ (24.1x70mm) is a powerful large caliber rifle cartridge developed by American gunsmith and weapon designer J. D. Jones of SSK Industries.
.950 JDJ cases are approximately 70 mm in length, and are based on a 20×110mm case shortened and necked up to accept the .950 in (24.1 mm) bullet. Projectiles are custom-made and most commonly weigh 3,600 grains (230 g) which is 8.2 ounces or over half a pound.
As its name implies, rifles chambered for the cartridge have a groove diameter of 0.950 in (24.1 mm). SSK received a "Sporting Use Exception" to de-regulate the rifles. Thus, in the United States, they can be purchased and owned like any other Title I rifle by an American citizen at age 18. The rifles use McMillan stocks and extraordinarily thick Krieger barrels bearing an 18 lb (8.2 kg) muzzle brake. Overall, depending on options, the rifles weigh from 85 to 120 pounds (39 to 54 kg) and are therefore only useful for shooting from a bench rest or heavy bipod. Despite the weight, recoil is significant and shooters must be sure to choose components (i.e., scopes and bipods) that can handle the abuse. The sheer size, weight, and power of these rifles make them rather impractical for hunting use. SSK only manufactured three of these rifles and as of 2014 no longer produces the ammunition.
By comparison, the 5.56×45mm NATO cartridge, used in the M16 rifle, produces between 1,200–1,300 ft⋅lbf (1,600–1,800 J), while the .308 Winchester, a favorite for hunters and medium-range police/military sniping, produces between 2,000–3,000 ft⋅lbf (2,700–4,100 J) depending on the load used. The ballistics of the .950 JDJ are more similar to that of the 20 mm autocannon round, which delivers approximately 39,500 ft⋅lbf (53,600 J). The muzzle energy of the .950 JDJ is comparable to the kinetic energy of a 2,800 lb (1,300 kg) automobile traveling at 20 mph (32 km/h).
In a 110 lb (50 kg) rifle, this will develop well over 200 ft⋅lbf (270 J) of free recoil energy. Shooting usually involves a heavy "lead sled" or similar shooting rest, and the rifle scope has significant eye relief to avoid injuring the ocular orbit.