Because stick-on wheelweights are essentially pure lead, a 50:50 mix of clip-on and stick-on weights already has half the antimony content of regular wheelweights. I would not expect that to shatter easily. Also, Veral Smith said he'd thought wheelweight alloy was fine for all bullet casting until he moved to Idaho and discovered there that they shattered on bone in the extreme cold they can get in some places in winter (down to -60°F some nights at higher altitudes, IIRC, with Boise getting average minimums of -20°F in January). I don't know what temperature he observed shattering it at, but it was likely a good deal colder than you were shooting in.
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