older 8mm mauser ammo was .318 and barrels matched... later a .323 was adopted and many mausers have a marking on the barrel to differentiate bore diameters so soldiers wouldn't use the wrong ammo in them.
my guess is that his g88 was a 318 bore and that yours is a 323 bore. the ammo you tried in his might have been slightly underbore and better suited to his barrel.
all guns lost in a tragic smelting accident.
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