Enact a law to engrave buyer's social security number on all new purchased ammunition. The engrave SSN will hold the buyer accountable for any harm inflicted on others caused by the buyer's ammunition, even if the buyer was not directly involve in the incident.
WIth higher accountability, the buyer will take extra precaution to proper store firearms and ammunitions. In addition, the engraved ssn will allow law enforcement to easily identify who purchased/used ammunition and act as a deterrent.