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Just because a rifle may be made on a Mauser 1898 action, it does not mean it is an “S-bore”!
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An “S-Bore” pertains to the larger of the 2 bore diameters associated with the 8×57 Mauser cartridge. The actual diameter of the early 8×57 cartridge was .318”. The early Mauser model 88 was chambered in this 8×57 with this bore diameter. Many actions prior to the 1898 model had the .318” bore. Some commercially made model 98s were also chambered with the smaller bore. Just because a rifle may be made on a Mauser 1898 action, it does not mean it is an “S-bore”!
The “S-bore” Mauser cartridge has a larger diameter bullet, which is .323” diameter. This is ONLY to be used in the larger bore of the German military rife adopted in 1898. The “S” was chosen because bullets used at that time had round noses. The Germans determined that better accuracy could be achieved with a pointed bullet, a “Spitzer” style. The added “S” suffix was to clarify the difference between the old 8×57 and the newer version for the military. The modern, commercially available 8mm Mauser ammunition has been manufactured for the “S-bore”. The early .318” bore ammunition is a rarity, but there are many rifles still in circulation, which can only fire the .318” bullet.
As one might expect, any attempt to fire a bullet meant for a .323” bore in a .318” bore will create extremely high pressures. These high pressures may create a catastrophic failure of the action, resulting in injury and possibly death to the shooter and/or anyone in the vicinity of the detonation.
One must be very cautious when evaluating old 8×57 rifles for this reason. The case dimensions governing headspace are the same for both cartridges. The only method for determining safety is to check headspace and physically measure the bore.
It is because of this confusion between the .318” bore and the .323” bore that any and all Mausers should be examined by a competent gunsmith before they are fired.