Gun store owners called it the "Trump slump." Sales of firearms slowed dramatically after the election of Donald Trump as president in 2016 allayed fears of a Democratic crackdown on gun owners. That trend has continued in recent weeks even with talk of gun control in Congress and among business leaders following the Feb. 14 massacre of 17 people at a Florida high school. In the past, gun massacres generally led to an uptick in sales as people worried a...