The famous London gunmaking firm of Jeffrey introduced a proprietary series of cartridges around the turn of the last century designed to be used in massive double-rifles on the largest and most-dangerous African big game. The climax of the series, the .600 Nitro Express, introduced in 1903, remained the largest rifle catridge ever commercially loaded until 1988. The Nitro Express designation was applied to recently developed (circa 1900) higher velocity (Express) cartridges, loaded with Nitro, i.e., nitrocellulose, i.e., smokeless powder.
The .600 Nitro Express was three inches long, and 6/10 of an inch in diameter. The factory loading was 120 grains of cordite, which propelled a 900 grain Full Metal Jacketed bullet at a muzzle velocity of 2050 feet per second. More recent loads are slightly reduced (cartridge companies fear those old rifles are getting on in years) to the equivalent of 100 or 110 grains of cordite, producing only 1950 or 1850 fps.
This gigantic round was designed for only one purpose: to stop a charging elephant at close range with a single shot. Needless to say, it was possible to accomplish the same desirable feat with smaller cartridges, featuring less recoil, and only a very small number of rifles were built in the original period chambered for the mighty .600 Nitro Express.
There was a very substantial revival of interest in collecting, and shooting, classic British double rifles over the last few decades, and some of the surviving companies like Holland & Holland began producing them again to custom order. Finally, in 1988, purely for the fun of surpassing the historical record, a .700 Nitro Express cartridge was created.
More recently, it seems some custom gunsmith was commissioned to produce a barrel for the interchangeable-barreled Thompson Contender chambered for the dreaded .600 Nitro Express cartridge. Picture installing a Ferrari engine in a go cart.