WASHINGTON — Never much known for restraint, Joseph R. Biden Jr. did not hold back during a presidential primary debate in 2007 when a voter asking about gun rights in a recorded video displayed a fearsome-looking semiautomatic rifle and declared, “This is my baby.”

Mr. Biden, then a Delaware senator in a dark-horse bid for the White House, shook his head. “I tell you what, if that’s his baby, he needs help,” he said. “I think he just made an admission against self-interest. I don’t know if he’s mentally qualified to own that gun.”

The candidate’s blunt, dismissive remark cheered one side of America’s long-polarized debate about guns and alienated the other. But it overlooked the salient reality that the rifle-toting voter was able to buy it legally even under a law that theoretically banned assault weapons and was co-written by Mr. Biden.

Five years later, that same type of weapon, a Bushmaster AR-15, is at the heart of a renewed national conversation about gun laws because it was used this month by the mass killer in Newtown, Conn. For Mr. Biden, now the vice president, the moment offers a second chance as he drafts a legislative response for President Obama that would reinstate his expired assault weapons ban, while also applying lessons from the last time around to make it more effective.