7.5×55mm Swiss

GP90/03 ammunition

The 7.5mm Swiss cartridge has been in Swiss Army service since 1889 in their Schmidt–Rubin Model 1889 rifles. Originally using PC 88 ("powder composition-88") rauchschwacher ("low smoke" - equivalent to "smokeless") cut tubular smokeless single-base powder relying on nitrocellulose as its propellant ingredient, it was known as the Gewehrpatrone 1890 (GP90). The GP90 was loaded with a paper patched lead hollow based heeled steel-capped round-nose bullet. Starting from the rear of the nose section the bullets were wrapped around by two turns of paper, much like cotton patches were placed around the bullet of a musket. This paper patching reduced metallic fouling of the barrel and was supposed to aid in the gas seal of the bullet.

It was discovered that the primer of GP90 ammunition was far too corrosive so it was updated in 1903 to the GP90/03 cartridge.

PC 88 square flake shaped propellant used in the GP 90/23 cartridge variant

Long after the discontinuation of the Schmidt–Rubin Model 1889 rifle in the Swiss armed forces, the GP90/03 round was updated in 1923 and called the "GP90/23", for use in shooting competitions. The GP90/23 dispensed with the paper patching around the bullet and was loaded with a full metal jacket (FMJ) non-heeled round-nose bullet. The PC 88 propellant shape was changed to square flakes. GP90, GP90/03 and GP90/23 ammunition was available in nine different variants.[2]

Swiss Army issue 10-round GP 11 pack

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 7.5mm Swiss round was updated to the completely non-corrosive Gewehrpatrone 1911 (GP 11). The bolt thrust of the GP 11 round is relatively high compared to many other service rounds used in the early 20th century. Besides being used in the Model 1911 and Schmidt–Rubin 1896/11 rifles, GP 11 ammunition was also used in the MG 11 machine gun, K11 and K31 carbines as well as in the Stgw 57 battle rifles. The cartridge saw extensive service until the early 1990s with the standard rifles of Swiss servicemen, and still sees use by Swiss Army reservists and sport shooters, of which there are many. Furthermore, it is still the standard ammunition for the MG 51 general purpose machine gun used on many Swiss armored vehicles, such as the Pz 87 "Leopard 2" tank and the MOWAG Eagle reconnaissance vehicle. In this role, it is usually belt fed with GP 11 full metal jacket and GP 11 tracer rounds.

The GP 11 cartridge used double-base powder combining nitrocellulose (gun cotton) with about 30% nitroglycerin as propellants. The 11.3-gram (174.4 gr) full metal jacket GP 11 boat tail spitzer bullet was when adopted an innovative bullet design. The GP 11 bullet contained a lead-antimony core and its jacket was made of plated steel or tombac. Depending on the year of production the plating was made of copper, brass, nickel or copper-nickel. The GP 11 cartridge cases were made of brass (72% of copper and 28% zinc) or, from May 1943 to January 1947, due to supply shortages in Switzerland, of aluminum or steel. The GP 11 bullet offered good aerodynamic efficiency and ballistic performance with a ballistic coefficient (G1 BC) of 0.505 to 0.514. At 780 m/s (2,559 ft/s) muzzle velocity the standard GP 11 ball bullet retained supersonic velocity up to 800 m (874.9 yd) (V₈₀₀ ≈ Mach 1.1) under ICAO Standard Atmosphere conditions at sea level (air density ρ = 1.225 kg/m³). Even by 2011 standards, 800 m (875 yd) typical effective range is quite remarkable for a standard military rifle round that is more than a century old.[3]

Maximum range with the GP 11 under Swiss chosen atmospheric conditions (altitude = 800 m (2,625 ft), air pressure = 649 mm (25.55 in) Hg, temperature = 7 °C (45 °F)) equaling ICAO Standard Atmosphere conditions at 653.2 m (2,143 ft) (air density ρ = 1.150 kg/m³) is acquired when the barrel is elevated 37° and is muzzle velocity dependent.[4]

The GP 11 bullet set off the militaries of countries like Germany, the United States and the United Kingdom at the onset of and after World War I to develop and field similar full metal jacket boat tail spitzer bullets to improve the maximum range and long range performance of the full metal jacket flat based spitzer bullet designs they used.

Since its introduction in 1911 the GP 11 cartridge featured some technical development. The projectile-seating in the cartridge case neck was improved by adding a cannelure on the bullet and crimping the case neck over time, resulting in 30 kgf (66 lbf) (1911), 50 kgf (110 lbf) (1929 modification), 100 kgf (220 lbf) (1942 modification) extraction force. In 1942 the square flake-shaped double-base propellant was replaced by tubular shaped double-base propellant. Further, there were minor developments regarding the projectiles, sealing and primers. GP 11 cartridges were mass-produced for the Swiss military in the ammunition factories in Altdorf and in Thun until 1994. The last 1994 produced ammunition had lot No. 349-94. Previously GP 11 was also produced in Rotenburg and Solothurn as other municipalities in Switzerland.

GP 11 is regarded as highly accurate and well-manufactured service ammunition. For Swiss military service ammunition the primer-type is Berdan. Berdan-primed ammunition is not easy for reloading previously fired cartridges.

In addition to the standard full metal jacket GP 11 rounds, specialty rounds were produced as well for the Swiss military. Armor-piercing steel-core 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 tips.[5]

When all modifications are combined GP 11 ammunition was available in a total of 40 different versions.[6]

Stripper clip with 7.5×55mm Swiss GP 11 cartridges

Due to the greater pressures produced by the GP 11 rounds, they are not fired in Model 1889 Schmidt–Rubin rifles.

The 7.5×55mm Swiss / GP 11 cartridge (designated as the 7.5 × 55 Suisse by the C.I.P.)[7] 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.

7.5×55mm Swiss / GP 11 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 rifling twist rate for this cartridge is 270 mm (1 in 10.63 in), 4 grooves, Ø lands = 7.51 mm (0.296 in), Ø grooves = 7.77 mm (0.306 in), land width = 3.75 mm (0.148 in), and the primer type is Berdan or large rifle.

According to the official Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (C.I.P.) rulings, the 7.5×55mm Swiss can handle up to 380.00 MPa (55,114 psi) P_max piezo pressure. In C.I.P. regulated countries every rifle cartridge combo has to be proofed at 125% of this maximum C.I.P. pressure to certify for sale to consumers.[7] This means that 7.5×55mm Swiss / GP 11 chambered arms in C.I.P. regulated countries are currently (2016) proof tested at 475.00 MPa (68,893 psi) PE piezo pressure.

Switzerland is not a C.I.P. member state and therefore does not recognize any C.I.P. rulings and proofed its military 7.5×55mm rifles chambered in the GP 11 version of their service cartridge at 150% of the GP 11 load pressure of 313.717 MPa (45,500 psi). This means a Swiss military proof test would be executed at 1.5 * 313.717 = 470.57 MPa (68,250 psi) and a C.I.P. proof test would be executed at 1.25 * 380 = 475 MPa (68,892 psi). Swiss 7.5×55mm GP 11 proof tests are therefore not recognized in C.I.P. member states in their turn.

Despite its nomenclature, the 7.5×55mm Swiss can use the same 7.62 mm (.308 in) bullets as conventional 7.62 mm (.308 in) 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 bullet may be used. However, most Swiss match shooters use standard GP 11 surplus ammunition, a testament to the quality of the factory loading of the GP 11 round.

While the scarcity of reloadable cartridge cases previously made the 7.5 mm Swiss problematic for US shooters, reloadable cases are easily produced by reforming .284 Winchester brass. Case rims are slightly undersized, but this presents no problems so long as the rifle's extractor is in good condition. This allows the handloader to produce a GP90 load that is safe for the Schmidt–Rubin Model 1889 rifle. Prvi Partizan has been producing new made brass for reloaders as well as loaded ammunition so supplies of reloadable brass are less difficult to obtain.