The cartridges in this German 68 mm cartridge "family" are, in the order of development:
Standard military 7.92x57 mm Mauser 98(k) rifles have to be adapted by a competent gunsmith to function properly with these magnum cartridges since they are longer and the cases have a larger diameter then 7.92x57mm Mauser cartridges. The 7.92x57mm Mauser cartridge is also known as the 8 mm Mauser or in Germany as the 8x57mm IS (IS stands for Infantrie Spitz). In adapted standard military Mauser 98k rifles the large 6.5x68mm cartridges are however praised for very smooth and reliable feeding.
The widespread availability of standard size Mauser 98 rifles and the fact that the .375 H&H Magnum cartridge and its necked down version the .300 H&H Magnum with approximately 72.4 mm case length were too long to fit in standard sized Mauser 98 bolt action rifles makes the shorter 6.5x68mm, 8x68mm S and .375 Hölderlin interesting chambering options.
World War II spoiled the commercial introduction and spread of the 6.5x68mm. The cartridge became popular after World War II due to its high performance and flat trajectory, when German hunters were allowed again to own and hunt with full bore rifles. The 6.5x68mm's performance also made it that hunters who had problems with handling magnum cartridge recoil stepped down to less powerful but adequate medium cartridges like the 6.5x57mm, 7.92x57 mm Mauser, 7x64mm (Brenneke) or .30-06 Springfield (also know as the 7.62x63mm in metric countries). Recoil sensitive shooters can fit an efficient muzzle brake to significantly reduce the amount of recoil. With the help of a muzzle brake, the 6.5x68mm's recoil is reduced to tolerable levels.
6.5x68mm maximum C.I.P. cartridge dimensions. All sizes in millimetres (mm).
Americans would define the shoulder angle at alpha/2 ≈ 14.53 degrees. The common rifling twist rate for this cartridge is 250 mm (1 in 9.84 in), 4 grooves, Ø lands = 6.45 mm, Ø grooves = 6.70 mm, land width = 3.50 mm and the primer type is large rifle magnum.
According to the official C.I.P. (Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives) guidelines the 6.5x68mm case can handle up to 440 MPa (63817 psi) 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.
When the 6.5x68mm was introduced in 1939 it probably held the title of fastest production cartridge on the market. Nowadays there are commercial cartridges on the market which offer even higher muzzle velocities.
Since there are not many factory loads available (RWS offers only 2 factory loads) and due to its good field reputation, the 6.5 x 68 is often used by reloaders. They have used this cartridge extensively to create powerful loads by handloading.
Factory rifles in 6.5x68mm normally have 250 mm to 280 mm (1 in 9.8 to 11 inch) twist rates, which are too long to adequately stabilize modern long heavy 6.5 mm bullets. This slow twist rate was chosen since the bullet jackets used in the 1930s were not strong enough to cope with the forces generated by the magnum muzzle velocities of the 6.5x68mm. The traditional 6.5x68mm twists nowadays inhibits its potential. To make the 6.5x68mm an awesome long-range cartridge, rifle barrels with a 200 mm to 230 mm (1 in 7.9 to 9 inch) twist rate are a better choice. Due to the large case capacity in relation to the 6.5 mm (.264 inch) caliber bore size the 6.5x68mm is very harsh on barrels. The 6.5x68mm typically wears out a rifle barrel in 500 to 1000 rounds. A lot of thorough barrel cleaning (after every 5 shots) and carefully avoiding long strings of shots help to minimize barrel wear. This makes this cartridge unpractical for most competition shooters who tend to fire a lot of rounds in practice to acquire and maintain expert long-range marksmanship.
Reloaders use the 6.5x68mm as a Jack of all trades long-range cartridge on all European game from fox, roe deer and chamois upwards to the big European game like red deer and moose. These people realized that bullets with different characteristics can be utilized to produce varying effects on game. As with all 6.5 mm cartridges, the big game hunting bullets used in the 6.5x68mm have comparatively high sectional densities for good penetration on suitable size game animals.
Wildcats are not governed by C.I.P. or SAAMI rules so wildcatters can capitalize the achievable high operating pressures. It is often reported that modern 68 mm RWS brass can tolerate up to 500 MPa (72519 psi) piezo pressure. Because the 6.5x68mm offers an exceptional sturdy, pressure resistant cartridge case that can relatively easily be reloaded with primers, powder and bullets and hence be reused several times it has become quite popular amongst wildcatters. With the German 68 mm magnum cartridge case as parent case wildcatters have created .25x68, .270x68, 7x68mm, .30x68, .338x68, .375x68 or .416x68 variants.