Virtually all surplus or commercially loaded 7.62x39 ammo will have a bullet that matches the nominal 0.311-0.312" specification for the cartridge.
Ruger, in a bid to expand the handloading options for Mini-30 buyers, designed compromise bore and chamber specs which allowed both mil-spec and 0.308" bullets to be used with safety and at least tolerable accuracy.
IIRC, there is a gradual taper from chamber to bore in the Ruger barrel to accomplish this, and it seems to work pretty well. While not noted for gilt-edged accuracy, the Mini-30 is capable of grouping at least as well as any other regular production semi-auto 7.62x39, with the possible exception of the scarce AR-15 clones so chambered.
I wouldn't worry about using mil-spec ammo in a Mini-30. The Ruger engineers (and the lawyers surgically grafted to them) designed it to be a non-issue.
FWIW, virtually all of the 7.62x39 that goes through my Mini is the same mil-spec generic ball that feeds my SKSes. No problems in either.