The centerpiece of a Democrat-led gun-control effort a plan to expand the system of background checks aimed at detecting buyers ineligible to own guns failed in a 54-46 vote, six votes shy of the 60 needed to advance.

WSJ: Background-Check Gun Measure Fails in the Senate

Gun-control advocates have helped highlight these flaws, but some also worry that focus on the system's holes will be used as an excuse to block legislation requiring universal use of the background check system.

WSJ: Systemic Gaps Hinder Background Checks for Gun Purchases