German military rifles made 1888 to 1905 were barrelled w/a .318” groove diameter; after 1905, w/.323” groove barrels. Many .318” rifles were rebarrelled w/the larger groove dimension & use a larger bullet. Between the wars, German made sporting rifles were .318” grooved, relating to Versailles restricitons. (as always, there were exceptions to all of the above) .
As others have written, if one is uncertain what groove diameter a particular rifle is, slug it/ have it slugged.
On a quick search just now I found a 150 gn Sierra jacketed bullet &Buffalo Arms had a 170 gn, both .318” & in stock. I’ sure there are cast le options out there as well. If you shoot too small a bullet through a rifle, it will be inaccurate. SOMETIMES, if the bullet is too large, it can take your head/face off!
shoot wisely.
Actually most of the above is incorrect . The P-88 ammo had a .3188 dia bullet . The later S ammo had a .321 dia bullet [ and may be what the op was seeing ] , and the later sS ammo had a .323 dia bullet . No German military rifle EVER used a .318 groove barrel . The Gew-88 barrel sizes were first year - .314 - .3208 , after 89 they went to .311 - .3208 to deepen the rifling , then in 1896 1/2 they went to the Z barrel that was .311 - .323+ . NO GEW-98 EVER had a .318 barrel as they were never used and after 96 1/2 .323 became the standard . The S ammo was designed to be used in the Gew-88 . 1905 has nothing to do with ammo or barrel size . German civilian barrels did use around a .307 - .317 [ + / - ] barrel with a .3158 dia bullet loaded shorter in a cartridge which was different from the P-88 ammo and was used pre WWI and had nothing to do with any treaty . All of this is clearly stated in original German documents , and found in real rifles and ammo .
Actually most of the above is incorrect . The P-88 ammo had a .3188 dia bullet . The later S ammo had a .321 dia bullet [ and may be what the op was seeing ] , and the later sS ammo had a .323 dia bullet . No German military rifle EVER used a .318 groove barrel . The Gew-88 barrel sizes were first year - .314 - .3208 , after 89 they went to .311 - .3208 to deepen the rifling , then in 1896 1/2 they went to the Z barrel that was .311 - .323+ . NO GEW-98 EVER had a .318 barrel as they were never used and after 96 1/2 .323 became the standard . The S ammo was designed to be used in the Gew-88 . 1905 has nothing to do with ammo or barrel size . German civilian barrels did use around a .307 - .317 [ + / - ] barrel with a .3158 dia bullet loaded shorter in a cartridge which was different from the P-88 ammo and was used pre WWI and had nothing to do with any treaty . All of this is clearly stated in original German documents , and found in real rifles and ammo .