In Corrosion Control classes they told us similar metals in contact cold weld because, in the case of bullet and case, the copper atoms in the bullet don't know they aren't the same as the copper atoms in the case, so they adhere at an atomic level, sharing electrons. Or so the Chief teaching the class said.
There is also galvanic corrosion, or welding together through electrical micro-voltage process which could be enhanced or encouraged by residual case lubes or cleaning detergents.
But, I don't know for sure. Never studied it in cartridges.
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