A 1.4" case, shortened from a .500 S&W case, with the rim trimmed so it will fit into a FA 83. A creation of Jack Huntington of JRH Advanced Gunsmithing in California. Powerwise, think of it as a .500 Wyoming Express or .50 caliber .500 Linebaugh... or for that matter, in the same power league as a .475 Linebaugh.
The BFR is now the only factory chambered gun in the .500 JRH. All others are custom jobs.