Wednesday's Senate Judiciary hearing was its third since the Newtown tragedy made gun violence a top-tier national issue. The Judiciary panel could begin writing legislation as early as Thursday, but that session is likely to be delayed until next week.
Numerous relatives and neighbors of victims of Newtown, as well as other shootings at Aurora, Colo., and Virginia Tech filled the large hearing room.
At one point, Feinstein played a video showing how a bump fire slide, a piece of equipment added to an assault weapon, allows it to rapidly fire many rounds of ammunition, much as a machine gun would.
"The need for a federal ban has never been greater," Feinstein said.
The panel's top Republican, Sen. Charles Grassley of Iowa, expressed sympathy for gun violence victim, but said existing gun laws are not being adequately enforced, including background checks designed to prevent criminals from getting weapons.
"We should be skeptical about giving the Justice Department more laws to enforce" when it's not enforcing current ones, Grassley said.
Grassley said he believed Congress will eventually take action on boosting penalties for illegally trafficking guns, on more adequately keeping guns from people with mental problems, and encouraging states do a better job of reporting mental health records of potential gun buyers to the federal background check system.
At one point, Sen. Lindsey Graham, R-S.C., used his questioning of Milwaukee Police Chief Edward Flynn to argue that the current background check system is not being well enforced, since only a handful of the roughly 80,000 people annually who fail those checks are prosecuted for filing documents saying they qualify to own the weapons.
Uncharacteristically for a Senate hearing, Flynn interrupted the senator, saying, "I want to stop 76,000 people from buying guns illegally," a reference to the gun purchases that the background check system blocked last year. "That's what background check does."