|Place of origin||United States|
|Designer||J. D. Jones|
|Parent case||20 × 102 mm Vulcan|
|Bullet diameter||0.950 in (24.1 mm)|
|Case length||4 in (100 mm)|
Loaded .950 JDJ cartridges are approximately the length of an empty .50 BMG casing (i.e., 4 in or 10 cm), and are based on a 20×102mm 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 bore diameter of 0.950 in (24.1 mm), which would normally classify them as Destructive Devices in the United States under the National Firearms Act of 1934. However, SSK sought and received a "Sporting Use Exception" to de-regulate the rifles. In the United States they can be purchased like any other Title I rifle by a person at age 18 with no felonies on their criminal record. 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 110 pounds (39 to 50 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. Thus, they are largely "range queens"—rifles that are brought to the range for a fun time, but not usually used for hunting or other "more practical" uses.
The cartridge propels its 3,600 gr (230 g) bullet at approximately 2,200 ft/s (670 m/s). This yields a muzzle energy of 38,685 ft·lbf (52,450 J) and a momentum of 154.1 Newton-seconds, about the same as a 20×102mm Vulcan round.
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 20mm 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 if an efficient muzzle brake is not used. This is far beyond the shoulder-firing capacity of nearly all humans, even without considering the difficulty of shouldering such a heavy rifle. Shooting usually involves a heavy "lead sled" or similar shooting rest, and the rifle is not held to the shoulder because of the severe recoil and possible injury. The rifle scope has significant eye relief to avoid injuring the ocular orbit.
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