JD Jones’ .950 JDJ cartridge is a generally described as the largest sporting rifle cartridge ever produced, producing more energy than even the 4-bore cartridges that match it in bore diameter. Only three of these rifles were made, and the original loading was a 2600 grain (168g) cast bullet moving at 2200 fps (670m/s), for a whopping 28,000 foot-pounds of muzzle energy (38kJ). There are some antitank rifles that produce more, but nothing comparable in both energy and bore diameter in the sporting realm.
The rifle, named “Fat Mac”, is built on a McMillan stock and action, and was originally produced on special order for a customer who wanted a really (really) big rifle that could use cast bullets. The cartridge cases were originally made from 20mm Vulcan cases trimmer down to a (mere) 70mm case length, with custom made lathe-turned cases replacing them when brass Vulcan cases became too difficult to source. The gun itself weighs a bit over 60 pounds, with nearly a third of that in the muzzle brake alone.
Under American federal law all firearms greater than .50 caliber must be granted a special sporting use exception status to be sold like any other Title 1, without further restrictions. This status is most commonly granted to shotguns as most shotguns are greater than .50 caliber. The presumably charismatic creator of this single shot rifle got that status granted for his creation.
Ian's video: [8:03]
On the range: [1:32]
Scott from Kentucky Ballistics is probably drooling over this.