8mm and 8x57js are the same round. You'll see the 8mm only wording alot on the US manufactured rounds, which by the way are underpowered.
You really want to look for 8x57 loads from Europe (S&B, HotShot, Norma, mil-surplus) for full power loads the rifle was designed for.
And...no such thing as a dumb question
Not being sure and firing the wrong ammo for a rifle..now that's dumb