The .255 Jeffery Rook is a rimmed centerfire cartridge.

It was loaded with both black powder and smokeless powders, usually with a 65 gr (4.2 g) lead solid or hollowpoint bullet at a standard muzzle velocity of 1,200 ft/s (370 m/s).

Jeffery & Co and originally designed for use in rook rifles for hunting small game and target shooting.

It gained an excellent reputation for accuracy and was widely used for target shooting before being superseded by the .22 Long Rifle as a miniature target round, and it was also chambered in some single-shot pistols.

Jeffery & Co rook rifle chambered in .255 Jeffery Rook on a number of his expeditions stating it "is often better than the shot-gun for collecting specimens and providing for the pot.