.50 GI cartridge
From Wikipedia...
The .50GI operates at pressures comparable to the .45 ACP, around 15,000 psi. Felt recoil is not unlike that of the .45 ACP. The .50GI has developed a reputation for accuracy, though this may be due to the high precision of the semi-custom and very expensive Guncrafter pistols themselves. In one test, the 300 grain (19 g) JFP (jacketed flatpoint) gave a 25-yard group of 2.24 inches, and the 300-grain JHP (jacketed hollowpoint) and 275-grain JHP gave a 25-yard group of 2.14 inches.
The penetration in gelatin (but not necessarily the kinetic energy) of the .50GI is significantly different than the .45ACP, as it was purpose built to have a recoil impulse and kinetic energy substantially less than the magnum .50 caliber rounds such as the .50 AE (semiautomatic) or .500 S&W Magnum (revolver). Factory loaded ammunition has a kinetic energy of around 500 ft·lbs.
The cartridge is rarely used in law enforcement or for personal defense due to limited availability of ammunition and guns chambered for the cartridge. Currently, the only commercial handguns available in this caliber are Guncrafter Industries' own Colt 1911 handgun variants and its Glock 20 / Glock 21 conversion upper, and Magnum Research chambers their BFR revolver in this caliber on a custom basis.