Re: Bullet diameter
Yes. If the bullets are consistent within their lot the difference between .3076 and .3081 is not sufficient to cause accuracy disparities, by itself. Specifically, if they are all .30760" and less than .00005" eccentric then they can be very accurate. There will be different engraving pressures(the larger diameter will not always give the greatest engraving pressure), start pressures and rise.
For die drawn jacketed bullets there will be a tolerance range given for the life of the die, i.e. a smaller bullet may simply be from a new die(s). Typically, for benchrest quality bullets this is .0001". Dies are expensive and no matter how a manufacturer says otherwise there are tradeoffs made for cost. Eccentricity is probably more important for accuracy than a basic diameter.