"I find it amazing that we go back and look at statements, many of them — most of them — taken out of context of 10, 20, 30, 35 years ago," he continued. "And it's like my going back and pointing out how Bernie voted against the Brady Bill five times while I was trying to get it passed when he was in the House, or how he voted to, you know, protect gun manufacturers — it's the only group in America you can't sue."

Sanders adviser Jeff Weaver previously told PolitiFact that, at the time, the candidate believed the Brady Bill constituted an example of federal overreach. In an interview last month with The New York Times editorial board, Sanders said that "the world has changed" since his votes against the measure, and acknowledged that "I certainly have changed" with regard to his opinions on gun control.

While Biden said Wednesday that Sanders has "made up for that" and "indicated that was past," the broadside represents a new front in a rapidly expanding conflict between the two chief competitors for the Democratic nomination. In the party's 2016 presidential primary, Sanders was attacked for his record on guns by rival Hillary Clinton, and he could face further scrutiny related to the issue as the 2020 campaign escalates.

The Brady Handgun Violence Prevention Act of 1993, signed into law by President Bill Clinton, mandated the establishment of the National Instant Criminal Background Check System, which the FBI launched in 1998. The Brady Law requires that all Federal Firearms Licensees contact NICS to run background checks on potential gun purchasers.

Potential buyers must enter their personal information on a Firearms Transaction Record provided by the Bureau of Alcohol, Tobacco, Firearms and Explosives — also known as ATF Form 4473. NICS uses the information on an ATF Form 4473 to determine a potential buyer’s eligibility to own a weapon. Most background checks are completed within a few minutes, according to the FBI.