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So you were using pure lead then? If temperature is too high they will fill the mold and shrink when cooled. Get yourself a thermometer and play with temp. You may also want to add a little tin.
Casting anything involves variables; certainly the temperature of the material comes into play, but so does the mold temperature. Too cold is usually more of an issue than too hot. A uniformly heated mold is bigger than a cold mold, a cold mold causes premature solidification, which will result in both undersized casting and poor finish. 6 cavity is a big mold, and uniform temperature cavity to cavity during a pour is difficult. This would explain the size and quality variation. A two cavity mold giving good bullets all the time is a wiser use of time that making a crap load, then having to sort and multiple coat some.
Always smarter to prevent a problem than to detect and sort once a problem occurs.