It depends, to some extent, on what kind of Mosin you're shooting. The Finns run about .310", with some as tight as .3085" These will generally do best with the special Lapua bullets that are nominal .308", but have some unique features. The soviet and Russian bores are .311" and above. You can certainly shoot .308" bullets out of them, but they won't be as accurate.
Also, most .30-30 bullets are designed for the .30-30, with crimp bands and flat noses. They don't have much point in a bolt action rifle, though they would expand really well at 54R velocities I expect.