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In a small town, if you live there, it might be wise to leave it alone.
An LEO who would take your ammo on false pretenses might also be the vindictive sort at the next opportunity.

But I'm not that sort either. I suspect at the time the friend was so happy that the police intervened in time and retrieved his property, that he was not inclined to press them on a minor point of little financial consequence.
But I'd probably at least call the chief and explain the situation, along the lines of "I can understand an officer not knowing all the laws, but ammo's expensive, and I'd prefer to keep what's mine."