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WASHINGTON — Republican senators opened the door a crack to discussing gun policy in the wake of the Las Vegas massacre, with lawmakers zeroing in on a modification the shooter used to fire hundreds of rounds into a crowd over nearly 10 minutes.

Congress hasn't acted after previous gun attacks, and many Democrats have given up hope for new gun laws. But the nature of the Vegas shooting is taking the discussion in directions where partisan lines are less clear for now.

The attack was especially deadly in part due to the shooter’s high rate of fire, which in video of the incident sounds like a machine gun.

Bureau of Alcohol Tobacco and Firearms special agent Jill Snyder said Tuesday night that the shooter in his hotel room had 12 "bump stocks," a legal aftermarket accessory that allows semi-automatic weapons to fire at rates approaching automatic ones. Snyder said investigators were still determining which weapons in the hotel room were used in the shooting.

While they've shown no interest in more sweeping gun restrictions, Republican senators suggested Tuesday that Congress should investigate modifications that in effect skirt the federal government's virtual ban on automatic weapons.

"I think it’d be a good time to have a hearing," Sen. Lindsey Graham, R-S.C, told NBC News. "Just find out, 'How does the technology work?' and is there a legislative solution."

Sen. Dean Heller, R-Nev., said he wanted to talk to gun makers about finding ways to prevent modifications to weapons.

"One of the concerns that I have is the ability to manipulate a semi-automatic rifle and turn it into a fully automatic rifle," Heller said. "There has to be a way to be able to stop this."

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Democrats seized on the issue and hoped it might break the partisan logjam over guns.

Sen. Dianne Feinstein, D-Calif., a key advocate of the assault weapons ban, which lapsed in 2004, called the sale of bump fire stocks a "loophole" that should be closed.

Sen. Claire McCaskill, D-Mo., who is facing a tough re-election in a red state next year, called on the NRA to do some "soul searching."

"I don’t know anybody who goes deer hunting that needs to retrofit a gun to fire hundreds of rounds per minute," she said. "It’s to slaughter people."

Fully automatic weapons, which fire as long as the trigger is held down, are no longer manufactured for civilian use and existing fully automatic weapons are hard to obtain and rarely used in crimes.

But YouTube is filled with clips of gun enthusiasts teaching "bump fire" shooting techniques for semi-automatic handguns and rifles, which use the natural recoil of each shot to depress the trigger and fire at much faster rates.

Some use specialty modifications that cost hundreds of dollars, while others achieve the effect by simply using rubber bands, their belt loops, or even just an altered grip. These items are often treated as a novelty by gun owners, since they make it harder to fire accurately.

The Bureau of Alcohol, Tobacco, Firearms (ATF) has given its blessing to manufacturers of bump stocks, since the modifications do not change the internal mechanisms of firearms and technically still require the trigger to be pulled once for each shot, though it presumably could change that rule.

The Las Vegas attack threatens to undermine longstanding arguments by gun rights activists that semi-automatic rifles are not "assault weapons," a label they say should apply only to automatic weapons. If a shooter can fire a semi-automatic so quickly as to resemble an automatic weapon, the line gets blurrier.

"There’s already a consensus in America that machine guns should be heavily restricted if not outright banned," UCLA law professor Adam Winkler, who authored a book on the gun debate, told NBC News. "We may see a real push to make those modifications illegal."

Even so, a sense of fatalism has overtaken some of the most vocal gun safety advocates on Capitol Hill.

"I lost my optimism after Sandy Hook when we still did nothing," said Rep. Adam Schiff, D-Calif., of the 2012 shooting at an elementary school in Newtown, Connecticut.

And any action by Senate Republicans would likely run into problems with their colleagues in the House, who tend to hail from more conservative districts and often have to worry about primary challenges.

Rep. David Cicilline, D-R.I., is working on a bill to ban the possession of bump stocks, but said he expects it to go nowhere.

"There are in excess of 100 pieces of legislation that would in some way reduce gun violence in America," he said. "I’ve seen nothing that would indicate a willingness to enact even the most modest gun safety proposals."

Asked who on the Republican side might be open to working with him, Cicilline paused for a moment to think before replying, "I can’t think of anyone."