Cars don't have a ballistic finger-print, looking at the analogy made in the OP.
My guess is that guns are destroyed because they have characteristic ballistic properties in their rifling and the police don't want to close a case and then have another shooting with the same ballistics taking place yet un related to the initial crime.
That is the only reason I can think of.
It is a shame though: getting them re-barrelled would be an option, assuming I have it right...
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