Steyr M95 bore diameter.

[Patrick Chadwick]

First, let me say that all old rifles need to be treated as individuals. Even if made to a specific standard. Please consider that most of the old milsurps that concern us were made long before CIP or SAAMi were set up!

For what it's worth, here are the bore and groove dimensions from the German "Masstafeln" = dimensional tables. These are the values used by proof houses over here, regardless of what you might have read in the English-speaking world. ANYTHING THAT YOU READ ELSEWHERE, ESPECIALLY WHEN GIVEN IN INCH DIMENSIONS, IS DERIVED INFORMATION AND SHOULD BE TREATED WITH APPROPRIATE CAUTION. The inch values given below are approximations!

I apologize for shouting, but it has already been correctly pointed out that derived information may contain errors.

8x57J - NOT the old Gew.88 bore. A derived standard for civil rifles only. See later correspondence. Note the use of J, not I, to avoid confusion with the number 1.
Bore / Groove 7.80 / 8.07 mm = 0.307 / 0.318

8x57JS - the "classic" Gew. 98 /Kar 98. S stands for "Spitzer" bullet, already mentioned in previous posts.
Bore / Groove 7.89 / 8.20 mm = 0.311 / 0.323

8x50R - the original Austrian Mannlicher miltary caliber. Most altered to 8x56R M30S
Bore / Groove 7.95 / 8.35 = 0.313 / 0.329

8x56R M89 Portuguese Kropatschek. Sometimes confused with the Hungarian 8x56 (below).
Bore / Groove 7.85 / 8.20 = 0.309 / 0.323

8x56R M30S - the later Hungarian M(odel 19)30 S(pitzer) caliber.
Bore / Groove 7.95 / 8.35 = 0.313 / 0.329

So you can see, there is no such thing as a simple "8mm caliber". Add in wear, refurbishment, and just plain Bubbaring, and it is clear that it is vital to measure all old milsurps before attempting to shoot them.