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What is the true bore of the .315 / 8x50R?

Is it the British system of the rifling lands, the usual groove depth system or ?

The bullet is heavier that the usual .311, .318 or .323 calibre projectiles.

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The original Austrian military barrel dimensions of the M88 – M95 straight pull Mannlichers are still the current CIP numbers. These prescribe 4-groove barrels with an 250 mm = 10” right hand twist. The bore and bullet diameters are a bit unusual by today’s practice, but designed by pre-1900 ideas for the use of the long, cylindrical bullets with large bearing surface. As the then infant smokeless powders were very fast burning, those long bullets slugged up on firing. Before firing, the 8x50R bullets had a diameter of 8.22 mm = .324”. To keep pressure at bay, the barrels had a bore/land diameter of 7.95 mm = .313” with 8.35 mm = .329” grooves. ( Note, these are the barrel dimensions adopted by Westley – Richards for their .318 some years later).When Austria and Hungary converted their Mannlichers to the 8x56R M30S cartridge, using lighter 208 gr pointed boat tail bullets that did not slug up, they increased bullet diameters to .330” to fit the existing bores. The same “fitting bullet to groove diameter” was done in 1904 by the German military when they switched from the 224gr .318” rn bullet to the 154gr .323” pointed Spitz bullet of the 8x57 IS.
The 8x50R chamber is peculiar too: There is no defined step from case mouth to barrel leade, but just a long straight taper from case shoulder to rifling. This allowed the Austrians to use cases of different lengths for special loads in the same chambers, f.i. the WW1 Alder incendiary and tracer loads with cases up to 54 mm.

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