The .375 Ruger (9.5×65.5mm) is a rimless, standard-length rifle cartridge designed for the hunting of large dangerous game. It is designed to provide an increase in performance over the .375 H&H cartridge, yet to be chambered in a standard length action rifle. The cartridge was designed by Hornady and Ruger in partnership and released commercially in 2007 and chambered in the Ruger Hawkeye African and the Ruger Hawkeye Alaskan rifles.
.375 Ruger Wikipedia
The .375 Ruger uses a unique case designed by Hornady and Ruger. The case is of a rimless design having the base and rim diameter of .532 in (13.5 mm) which is the same diameter of the belt on belted magnum cases based on the .300 H&H Magnum and .375 H&H Magnum. This allows the cartridge to have a greater case capacity than a belted magnum case given cases of equal length. As Ruger intended the cartridge to be chambered in standard length bolt-action rifles the case length was kept to 2.580 in (65.5 mm) which is only .04 in (1.0 mm) longer than the .270 Winchester case. The maximum overall length of the cartridge is 3.340 in (84.8 mm) which is similar to the maximum overall length to standard length cartridges such as the .338 Winchester Magnum or the .30-06 Springfield. Unlike Remington Ultra Magnum cartridges, the Ruger Magnums can be chambered in standard length bolt-action rifles. This allowed Ruger to chamber the cartridge without extensively redesigning their M77 rifle to adopt them to the new Ruger cartridge.
While the .375 H&H Magnum is longer than the .375 Ruger, the latter cartridge has a greater case capacity than the Holland & Holland cartridge. This is due to the .375 H&H Magnum having a long tapered body while the .375 Ruger follows modern cartridge designs in that it has very little taper and a sharper shoulder. The case capacity of the .375 Ruger is 99.0 gr. of water (6.43 cm3) while the .375 H&H Magnum has a case capacity of 95.3 gr. of water (6.19 cm3), an increase of 4%.
Cartridge standards for the .375 Ruger were issued by SAAMI in June 2007. SAAMI recommends a 6 groove barrel having a bore Ø of .366 in (9.3 mm) and a groove Ø of .376 in (9.6 mm) with a groove width of .115 in (2.9 mm). The recommended rate of twist is one revolution in 12 in (304.8 mm). Recommended maximum pressure for the cartridge is 62,000 psi (4,300 bar).
Currently Hornady and Double Tap manufacture ammunition for the .375 Ruger cartridge. The Hornady superformace ammunition drives a Hornady 270 gr (17 g) SP-RP bullet at 2,840 ft/s (870 m/s) and the 300 gr (19 g) DGS and DGX bullets at 2,660 ft/s (810 m/s). The Double Tap achieves 2825fps - 4700 ft./lbs. with a 270 grain Barnes TSX from a 23" bbl. Ruger 77 African. The.375 Ruger's slightly greater case capacity, and the "short fat" cartridge efficiency lead to increases in the neighborhood of 150 fps over the H&H cartridge. Their capabilities remain essentially comparable.