When I cast some plastic balls , I will be certain to cool them slowly.What probably happened is that the water was cold enough that it caused them to cool too quickly from the outside in. As it cooled at a quicker rate, the lead on the outside contracted, sucking in the lead on the inside, which pulled a vacuum and made a void on the inside of the bullets. Of course, given the normal volume of lead, but with a void in the center, it takes up more space and you end up with a larger diameter than if you had allowed it to cool more slowly. I bet if you sawed some of them in half you would be able to see the void.
I make (or more accurately chemically modify) plastic for a living, and out extruder runs strands of plastic through a chilled water bath to cool them off before running them through a cutter that then pelletizes the strands. With plastics that swell a lot when molten, the strands look almost like straws because the outside has cooled so quickly that they created enough vacuum while shrinking in the chilled water bath that they pulled a void in the center.
Your last sentence is the conclusion I was alluding to in my post.You didn't change the dimensions of the bullets. Even a small trace of antimony, arsenic, or a few other elements that don't exist in "solid solution" in lead at room temperature are in solid solution in hot bullets right out of the mold. Chilling them quickly traps it in that state, but it's unstable. The antimony, etc. quickly crystallizes out of the "solution," but at the low temperature the atoms can't diffuse quickly enough to form their normal relatively large crystals and form many more tiny crystals. Those pin in place layers of the lead matrix that slide across each other when deformed, making it much harder. Harder bullets are harder to push into rifling than softer ones.
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