|Place of origin||England|
|Case type||Rimless, Bottleneck|
|Bullet diameter||.505 in (12.8 mm)|
|Neck diameter||.535 in (13.6 mm)|
|Shoulder diameter||.600 in (15.2 mm)|
|Base diameter||.640 in (16.3 mm)|
|Rim diameter||.640 in (16.3 mm)|
|Rim thickness||.065 in (1.7 mm)|
|Case length||3.150 in (80.0 mm)|
|Overall length||3.850 in (97.8 mm)|
|Case capacity||178 gr H2O (11.5 cm3)|
|Rifling twist||1 in 16 (406 mm)|
|Primer type||.254/Kynoch#40/Large Rifle|
|Maximum pressure||39,000 psi (270 MPa)|
|Test barrel length: 24 inches (610 mm)|
The .505 Gibbs cartridge was designed by George Gibbs in 1911. The cartridge was originally known as the .505 Rimless Nitro Express. The C.I.P. refers to the cartridge as the 505 Mag. Gibbs in their publications. It is a .50 caliber (12.8 mm) rimless bottlenecked cartridge intended for magazine-fed rifles.
The .505 Gibbs has a case capacity of 178 grains (11.5 g) of water. This cartridge was originally loaded with 90-grain (5.8 g) of cordite and 525-grain (34.0 g) bullet at 2,300 ft/s (700 m/s) for 6,166 ft⋅lbf (8,360 J) of kinetic energy. While the .505 Gibbs has a greater case capacity than most modern cartridges, it is loaded to lower pressures. The C.I.P. recommends a pressure of 2,700 bar (39,000 psi) for the cartridge. As .505 Gibbs was intended for hunting dangerous game in a tropical environment and due to the temperature sensitivity of cordite the lower pressures provided a greater safety and reliability margin.
The .505 Gibbs has a unique bullet diameter of .505 in (12.8 mm) while most other .50 caliber bullets have diameters of .510 in (13.0 mm). Barnes Bullets and Woodleigh Bullets are a few of the bullet manufacturers who produce component bullets for reloading in this caliber. Woodleigh Bullets does not recommend impact velocities of over 2,200 ft/s (670 m/s) for their .505 caliber 600 gr. Weldcore bullets.
George Gibbs Ltd. (England) continue to manufacture .505 Gibbs rifles; other premium dangerous game rifle makers, such as Hartmann & Weiss and Westley Richards build expensive but thoroughly reliable .505 Gibbs rifles on Mauser 98 actions.
As of 2019, Federal, Kynoch, Norma, and Swift are offering factory loaded ammunition in .505 Gibbs. Bullets for reloading are available from Barnes, Cutting Edge Bullets, North Fork Bullets, and Woodleigh Bullets.
The .505 Gibbs is one of the most voluminous cases designed. The large volume was required as the cartridge was designed to burn cordite as its propellant. The C.I.P. has published specifications for the cartridge.
The C.I.P. recommends that commencement of rifling begin at 8.97 mm (0.353 in). Bore diameter is given as 12.55 mm (0.494 in) and groove diameter is 12.80 mm (0.504 in). The C.I.P. recommends a five groove barrel contour with each groove having an arc length of 5.33 mm (0.210 in) and a twist rate of one revolution in 406 mm (16.0 in). The recommended pressure for the .505 Gibbs is 2,700 bar (39,000 psi).
The .505 Gibbs is a niche cartridge designed for hunting heavy, thick skinned dangerous game animals, such as cape buffalo, elephant, and rhino.