Skychief, Regardless of whether you're casting RB's or say, .30cal. rifle bullets, there is usually some variation in the weight and occasionally diameter of the castings (assuming the alloy is constant). Some of this can be reduced by pouring technique and maintaining a fairly constant temperature (I use a casting thermometer for this,) You can further reduce weight variation by weighing a sample (~25 - 40), calculating the mean and standard deviation, and retain only those RB's (or other types) within that range.* That way you can't fault the projectile for a flyer or missed shot. Btw, I have a Shiloh .495" RB mold whose cavities produce RB's that differ in both diameter and weight. By way of contrast, I also have a Saeco 3 cavity mold (#315), whose droppings are remarkably consistent.
*You can be even more selective if the application is more critical, going to 1/2 a SD if you must.