Bureaucracy is a bigger issue. All guns registered in the US are required by law to have their serial numbers recorded only on paper, not on computers. This means that even if you know the serial number of a gun used in a crime, it can take weeks or months to trace it to a purchaser. The reverse search (find all serial numbers associated with a particular person) is basically impossible.

Source: https://www.gq.com/story/inside-federal-bureau-of-way-too-ma...