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I've been playing around with different alloys with varying amounts of copper and also tin. I noticed a marked difference in expansion with different alloys, independent of velocity (which is unknown). The catch medium was rubber grinds. Bending of the boolits is likely caused by internal voids - I say this because some have popped and the bend was in that direction.

Some alloys seem to just flatten out sideways forming a 'disc' while others curl back around the shank and yet a third form is oval. Those two on right were different velocity with the same alloy and both have near 100% weight retention so the shape of the bottom one is not from metal loss.

Can anyone shed some light on this? Which alloys behave in which way? Similar observations?