does a mold predictably drop bullets larger/smaller as lead content is increased in the alloy?   for example:  cast with 30:1 and then cast with 20:1 and then compare dropped bullet diameter.    I only have 20:1 so I can't experiment to get my answer.  thanks

Not at all alloys will affect bullet size. Most tin : lead alloys will show small differences but it will only be noticeable when you go from say 40:1 t0 10:1.  You will get more variation with temperature variations.
Alloys with antimony will shrink very little Linotype and lyman No2 don't shrink hardly at all. 

40 Rod

      Main reason for change of diameter with alloy change is from the differences in melting points of the alloys.   If the alloy solidifies at a lower temperature, by the time it cools to room temperature it will have shrunk less than one which solidifies at a higher temperature.    These are measurable changes, but not something that it pays to worry about a lot.

CHRIS
RGChristensen

The material that the mold is made of and the temperature that the mold is, when it cast a bullet, make a difference on as cast bullet size, as well as the alloy used.

This data is based on the mold being cast iron and the mold temperature at 375F. The internal dimension of the mold, in this case, is cut to cast a .322 diameter bullet, in 20/1 with the above, mold material and temp specs.

It also gives what diameter the bullet will be, if cast of different alloys, again in a cast iron mold and the mold temp of 375F.

Frank