I wouldn't have guessed this, either, but have experienced it and it still kind of amazes me....I never would have guessed that different size bullets will come from the same mold depending on the lead used. I think I may just stick to buying my bullets for the time being.
The bullet diameters and weights presented in this list are based on the use of Taracorp’s Lawrence Magnum bullet alloy (2% tin, 6% antimony, 1/4% arsenic, 91.75% lead).
You should be aware that bullet diameters and weights will vary considerably depending on the lead casting alloy used. This variation can be as much as 1/2% on the diameter, and 8% on the weight among the most commonly used casting alloys. For example, a .358-158 grain bullet might show a diameter variation of .002", and a 13 grain difference in weight.
In order to provide a hard bullet, and due to the availability of high antimony content alloys such as scrap wheel weights and type metal, many casters are using high antimony content alloys. SAECO moulds work well with these high antimony content leads, but you should be aware of the variations they may produce.
Of the most commonly used alloys, wheel weights (.5% tin, 4% antimony, 95% lead) will produce bullets having the smallest diameter and heaviest weight, with such bullets running approximately .3% smaller in diameter and 3% heavier than bullets cast with Taracorp's metal. Linotype will produce bullets with the largest diameter and lightest weights. This alloy will produce bullets approximately 1/10% larger and 3% lighter than Taracorp. Other alloys of tin and antimony, with antimony content above 5%, will produce bullets with diameters and weights falling between those cast from wheel weights and linotype.
Alloys containing little or no antimony will cast considerably smaller than wheel weights and in some cases will produce bullets too small for adequate sizing. Within the limitations given above, the weight and diameter of a cast bullet can be adjusted by varying the alloy’s antimony content.
The size and weight of bullets of a given alloy will also vary according to casting temperature. Higher temperatures will result in greater shrinkage as the bullet cools, thereby producing a slightly smaller and lighter bullet than one cast of the same alloy at a lower temperature.