is the "Z" in a circle ?,, if so it means your barrel is a czeck barrel and is prolly as you say .318 .
if your reciever has the "s" stamp it just means that the chamber was opened slightly to accept the .323 spitzer bullet which will allow for the 8x57js round ,your barrel in my opinion would be safer to shoot the 8x57j size and stay away from military turk ammo its hot stuff and the gew88 wasn't really meant to take those kind of pressures.
i cast for my 88 and keep to under 1800 fps and below38,000 psi.
YMMV. have a good one .
