What kind of commercial ammo, specifically, did you try? U.S. made 8mm ammo is horribly downloaded out of fear that some idiot will try to fire it in an old rifle with a J bore (.318) rather than the newer JS bore (.323). Fortunately, Europeans are not so concerned with such things and European commercial 8mm ammo like S&B, Norma, and Prvi Partizan are not under loaded. If all you tried was U.S. made fodder, you might consider trying some European ammo to see if your accuracy doesn't improve.
Also, Turk ammo uses, IIRC, ~149gr bullets. While this was the original weight for the cartridge, the Germans found that it produced an undesireable level of muzzle flash when fired in the shorter barrel of the K98 and wound up standardizing on a 196-198gr bullet as did most other users of 8mm Mauser. As such, most commonly encountered 8mm Mauser rifles will have their sights regulated for the heavier bullet and the lighter Turk ammo will often shoot to a different POI.
While there is still a bit of 8mm Mauser surplus out there, it's rapidly drying up and much of what's left is the "bottom of the barrel" insofar as quality is concerned. If you can handload or find a commercial loading that your rifle likes, you'll have a much more reliable source of ammo than you would relying on surplus.
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