WASHINGTON — President Joe Biden celebrated the adoption of a federal rule in August that cracked down on the online sale of untraceable components for weapons known as ghost guns as a major step in stemming gun violence. But the rule has done little to stop the sale of key parts used to make deadly homemade firearms, according to officials and gun control groups. The rule, among the most significant executive actions Biden has taken on a core campaign promise, clarified the definition of a firearm under federal law to better regulate modern semi-automatic weapons. That in turn paved the...