05 November 2009
There's no magic to it, Tim. Think about it, the cavity does not change, so if you fill it fully and properly the resulting bullets should be exactly the same size and thus the same weight. Most times a double cavity mould will show a slight difference between cavities, because of normal tolerances in manufacture. That can't happen with a single cavity so the bullets will almost always be more consistent than bullets from a double cavity mould. As might be expected, the variance is bigger with multi cavity moulds, and I have a six cavity 45ACP mould which has one cavity that cast a full thousandth bigger and one grain heavier than the others.
The real challenge is to get that consistency from different batches of alloy. You can do that with virgin lead and antimony by accurate weighing, but it is near impossible with scrap because the content is unknown. But it is possible to get within half a grain each way batch to batch even with scrap.
Note that 1% variance of antimony ( eg 10% instead of 11% ) changes weight of a 500 grain bullet 2 grains, so accurate blending is important.
Having said all that, precise weight consistency is not the most important factor in accuracy. Other factors like bullet design, fitting the rifle, loading techniques, are bigger factors.