The .333 Jeffery also known as the .333 Jeffery Rimless Nitro Express or as the .333 Rimless Nitro Express is a rimless bottlenecked cartridge. It was developed by the W.J. Jeffery & Co in 1908 as a counterpart to the .333 Jeffery Flanged and was intended for use in magazine rifles.
The .333 Jeffery was designed for hunting medium game in Africa and India where it gained a following. While the cartridge is not intended for dangerous game, the cartridge was successfully used against these game species due to the excellent sectional density of the 300 gr (19 g) bullet. The penetrative capability of this bullet was a reason for its use as a counter sniper rifle cartridge between 1914 and 1918 as it could penetrate the German sniper shields before reaching the snipers.
The .333 Jeffery was originally loaded with cordite. It is loaded to a slightly higher pressure and performance level than its rimmed counterpart. The .333 OKH cartridge and the .318 Westley Richards cartridges provide similar performance levels as the .333 Jeffery.
Kynoch offers the loaded ammunition for the cartridge. They offer a 250 gr (16 g) bullet at 2,400 ft/s (730 m/s) and a 300 gr (19 g) bullet at 2,150 ft/s (660 m/s). Brass of new manufacture can be obtained from Bertram Brass of Australia and Kynoch.