The 7.5x55mm Swiss or GP11 (or unofficially 7.5x55mm Schmidt Rubin) is a cartridge developed for the Swiss Army by Eduard Rubin for rifles based on Rudolf Schmidt's action design. The ammunition used by the Schmidt-Rubin Model 1889 rifle was one of the first to use 7.5 mm copper jacketed rounds similar to those used today. The 7.5x53.5mm round designed by Colonel Rubin was revolutionary in that most of the bullets used in Europe at the time were around 12.7 mm as opposed to 7.5 mm of the Schmidt-Rubin ammunition. Strangley enough the round was "paper patched" meaning the actual bullet was surrounded by a piece of paper, much like cotton patches were placed around the bullet of a musket. Paper patching the round was supposed to aid in the gas seal of the bullet.
7.5 mm Swiss cartridges have been in Swiss Army service since 1889 in their Schmidt-Rubin Model 1889 rifles. Originally using a semi-smokeless powder the cartridge was known as the Gewehrpatrone 1890 (GP90). It was discovered that the primer of GP90 ammunition was far too corrosive so it was updated in 1903 to the GP90/03 cartridge.
GP90/23 the final cartridge variant for the Model 1889 rifleEdit
Long after the discontinuation of the Schmidt-Rubin Model 1889 rifle in the Swiss Army the GP90/03 round was updated in 1923 and called the GP90/23. The GP90/23 discarded the paper patching around the bullet and the semi-smokeless powder was replaced by smokeless powder propellant.
In 1911 the metallurgy and bolt design in Swiss military rifles had advanced enough that a more powerful cartridge could be used in the Model 1911 rifles and Schmidt-Rubin 1896/11 rifles. The 7.5 mm Swiss round was updated to the completely non-corrosive Gewehrpatrone 1911 (GP11). Besides being used in the Model 1911 rifles and Schmidt-Rubin 1896/11 rifles, GP11 ammunition was also used in the K11 and K31 carbines as well as in the Stgw 57battle rifle. The cartridge saw extensive service until the early 1990s and is now phased out, but nevertheless still sees use by Swiss Army reservists, albeit rarely.
The 11.3 gram (174 grain) Full Metal Jacket GP11 bullet offered good aerodynamic efficiency and ballistic performance with a ballistic coefficient (G1 BC) of 0.505 to 0.514 (ballistic coefficients are somewhat debatable). At 780 m/s (2560 ft/s) muzzle velocity the standard GP11 ball bullet retained supersonic velocity up to 800 m (875 yards) (V800 ≈ Mach 1.1) under ICAO Standard Atmosphere conditions at sea level (air density ρ = 1.225 kg/m³). Even by contemporary (2007) standards 800 m (875 yards) typical effective range is quite remarkable for a standard military rifle round..
GP11 is regarded as highly accurate and well-manufactured ammunition. Due to the increased availability of former Swiss K-31 rifles on the civilian market, a number of cartridge manufacturers, including Prvi Partizan and Hornady, now produce 7.5x55mm rounds in FMJ, soft-tip, and ballistic tip configurations.
In addition to the standard Full Metal Jacket GP11 rounds, specialty rounds were produced as well. Armor-piercing steel-cored rounds can be identified by their violet bases. These rounds can easily pierce 5 mm (0.2 in) of steel plate at 500 m (550 yards). Tracer rounds burn out to 800 m (875 yards), and can be identified by their red bases.
Despite its nomenclature, the 7.5x55mm Swiss can with proper care use the same 7.82 mm (.308 in) bullets as conventional Western 7.62 mm (.30 calibre) cartridges that have slightly wider land and groove diameters. This allows for ease of handloading and custom competition or hunting loads, as nearly any .308 diameter lead core bullet may be used. However, most Swiss match shooters use standard GP11 ammunition, a testament to the accuracy of the GP11 round.
The 7.5x55mm Swiss cartridge has 4.22 ml (65 grains) H2O cartridge case capacity. The exterior shape of the case was designed to promote reliable case feeding and extraction in bolt action rifles and machine guns alike, under extreme conditions.
7.5x55mm Swiss maximum C.I.P. cartridge dimensions. All sizes in millimeters (mm).
Americans would define the shoulder angle at alpha/2 ≈ 30.5 degrees. The common riflingtwist rate for this cartridge is 270 mm (1 in 10.63 in), 4 grooves, Ø lands = 7.51 mm, Ø grooves = 7.77 mm, land width = 3.75 mm and the primer type is large rifle.
Switzerland is no C.I.P. member state and hence does not recognize any C.I.P. rulings and proofed its military 7.5x55mm rifles chambered in the GP11 version of their service cartridge at 150% of the GP11 load pressure of 313.717 MPa (45500 psi). This means a Swiss military proof test would be executed at 1.5 * 313.717 = 470.57 MPa (68250 psi) and a C.I.P. proof test would be executed at 1.25 * 380 = 475 MPa (68892 psi). Swiss 7.5x55mm GP11 proof tests are therefore not recognized in C.I.P. member states.