Generally it's only after running the trace on a gun that's already been involved in a crime that it gets discovered. Sometimes it's thrown on as a pile-on charge in those cases and I guess sometimes they just don't bother using stretched-thin prosecutorial resources on someone who's already going down for whatever felonies got a trace run on the gun in the first place.
Hunter Biden's situation was unusual in that he was essentially dimed out by a concerned significant other, absent the gun being involved in any actual crimes.
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