Most widely debated is the variance of internal barrel dimensions and bullet diameters of early 7.9mm arms and ammunition. The topic often makes use of the terms J-bore, Z-bore, S-barrel, J-bullet and S-bullet. This writer has reviwed at least eight different explanations over the years and no two are exactly alike. Let's look at a few facts and see if we can figure out what was going on.
The first fact is that bore diameter (hole reamed in the barrel before rifling) for the 7.9mm cartridge has always been 7.9mm (3.11 inch). Why some call it an 8mm is unknown. The M88 cartridge used an 8.1mm (.3189 inch) diameter bullet (J-bullet). Rifling depth for the M88 barrel was 0.1mm (.0039 inch). Simple addition now reveals that barrel groove diameter (J-bore) and bullet diameter were identical at 8.1mm.
When M88 ammuntion with its long, hard jacketed bullet pushed by the new smokeless powder was fired in these barrels, the rifling quickly wore away and barrels ruptured, split or just plain blew out in various locations near the breech. Such was not desirable. Failures were blamed on the bullet being too large in diameter, though erosive, unstable powder was more likely at fault.
As a remedy, rifling groove depth was increased from 0.1mm to 0.15mm. We now have a groove diameter of 8.2mm (0.323 inch). Neither bore nor bullet diameter were changed. These barrels had a Z stamped on them. <my note: we are still talking about the M1888 Mauser rifle> Gas leakage past the bullet stopped the barrel blow-outs and deeper rifling gave longer barrel life. Remember, Germany was after better field results from the average soldier's rifel, not smaller bench-rest groups.
When the 7.9mm S (Spitzgeschloss or pointed bullet) was adopted in 1903, its bullet was 8.2mm (0.323 inch) in diameter (S-bullet), very pointed and only 154 grains in weight. Velocity is given as 2,930 fps. Smokeless powder employed was of a type using deterrant coatings to control burning rate. Gas leakage past the bullet was no longer necessary. Bullets were shorter, which reduced jacket fouling and rifling wear.
New barrels on the strong Model 98 action rifles were chambered for the S-cartridges. Weaker Model 88 rifles having the larger Z-bore were rechambered for the S-cartridge by opening the chamber neck diameter and freeboring to further relieve pressure. These barrels received an S stamp (S-barrel). <My emphasis>.
