The .454 Casull (pronounced Ka-Sool) is a firearm cartridge, developed in 1957 by Dick Casull and Jack Fulmer. It was first announced in November 1959 by Guns and Ammo magazine. The basic design was a lengthened and structurally improved .45 Colt case. .45 Colt cartridges can fit into the .454's chambers, but not the other way because of the lengthened case (very similar to the way .38 Special cartridges can fit into the longer chambers of a .357 Magnum and .44 Special cartridges can fit into the longer chambers of a .44 Magnum). The new Casull round uses a small rifle primer rather than a pistol primer, because it develops extremely high chamber pressures of over 60,000 CUP (copper units of pressure, not to be confused with pounds per square inch) (410 MPa), and a rifle primer has a significantly stronger cup than a pistol primer. The .454 Casull can deliver a 250 grain (16 g) bullet with a muzzle velocity of over 1900 feet per second (580 m/s), developing more than 2000 ft·lbf (2.7 kJ) of energy, although energy levels from common .454 revolvers with 7-8 inch barrels are typically somewhat lower (1,600-1,700 ft·lbf). The round is primarily intended for hunting medium or large game and metallic silhouette shooting.
The cartridges were originally loaded with a triplex load of propellants, which gave progressive burning, aided by the rifle primer ignition, resulting in a progressive acceleration of the bullet as it passed up the barrel.
The recently introduced .460 S&W Magnum cartridge has the same diameter as a .45 Colt or .454 Casull, and therefore revolvers chambered for it will also chamber the .454 Casull and .45 Colt. Until the introduction of the .460 Smith and Wesson Magnum, and the .500 S&W Magnum, the .454 Casull was the most powerful commercially produced handgun round on the market, significantly eclipsing the newer .50 AE in both power and availability.