WASHINGTON — In what could be the biggest expansion of federal background checks in decades, the Biden administration is moving to end the controversial “gun-show loophole.”   

“This single gap in our federal background check system has caused unimaginable pain and suffering,” said Vice President Kamala Harris in a call with reporters.

On Thursday, the Justice Department will submit a new 466-page regulation to the Federal Register outlining that the Bureau of Alcohol, Tobacco,…