A .577 caliber cartridge is a bullet used by professional guides and big game hunters for the express purpose of stopping a charging animal. According to Mike Schoby, who writes for Cabela's World's Foremost Outfitter, the .577 is a 750-grain bullet with a muzzle velocity of 2,050 feet per second.
Wikipedia details the history of the .577 caliber rifle. The Nitro Express is the first gun of this type, and it was produced in 1898 by a British company called Holland & Holland. It was carried by famous ivory hunters of the early 20th century such as James Sutherland, Count Alfred Józef Potocki, Ernest Hemingway and Stewart Granger. However, for these hunters it was strictly a backup weapon to be used in case of emergencies.