I recently pulled the bullets from 50 rounds of WWII-vintage .303 Britishicon ammunition because the cases were clearly too badly deteriorated to be safe. They are from multiple manufacturers (including some Canadianicon rounds). All the bullets have a noticeable cannelure, most have vertical grooves in the cannelure. What I'd like to know is this, after pulling the bullets, there was a slightly sticky, black substance in the cannelure and extending above and below it. What is this stuff?

Thanks in advance!
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