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The anti-gun violence effort was conceived by students at the high school in Parkland, Fla., where 17 students and staff were killed in a shooting on Feb. 14, 2018.
Here’s a look at gun-related fact checks we’ve done since the shooting
Democratic Montana Gov. Steve Bullock, a potential presidential contender in 2020, said in Milwaukee: "A quarter of our guns are sold outside of the background checks."
The latest study that surveyed gun owners on the topic found that, among gun owners who had acquired a gun in some way within the previous two years, 22 percent had done so without a background check.
That’s close to Bullock’s figure. But it takes into account people who acquired guns either by purchasing them, or by simply receiving them, for example as a gift.
Among those surveyed who had purchased a gun within the previous two years, only 13 percent said they had done so without a background check.
Also, U.S. Sen. Tammy Baldwin, D-Wis., said 97 percent of gun owners support universal background checks.
Baldwin’s claim accurately cited a figure in the latest national poll, but it applied to households in which there was a gun, not to gun owners per se.
U.S. House Speaker Paul Ryan, a Janesville Republican, said we "have laws on the books designed to prevent people with mental illnesses from getting firearms."
A federal law, and some state laws, do prohibit people adjudicated as "mentally defective" or involuntarily committed to a mental health facility from possessing a gun. But experts say that standard includes people who do not pose a danger to others. And it does not account for a much larger set of people who might be dangerous but have not been diagnosed with, or treated for, a serious mental illness.
How many guns?
Kevin Nicholson, one of two Republicans running in 2018 for Baldwin’s seat, said: "There are more guns in this country than there are people."
There is no official count of the number of firearms in the United States. In one estimate, for 2009, the number of guns exceeded the population. But the most recent estimate cited by gun-statistics experts puts the figure at 265 million guns as of January 2015, when the population was 320 million.
How many school shootings?
University of Wisconsin-Madison alumnus Jeff Greenfield, a TV journalist and author, said: "In the rest of the world, there have been 18 school shootings in the last twenty years. In the U.S., there have been 18 school shootings since January 1."
By one count widely cited in the news media, there had been 18 incidents at that time in which shots were fired inside or outside of a school or university building in the United States so far in 2018. But only three involved a mass shooting. And the count included two suicides, three accidental shootings and nine incidents in which there were no fatalities or injuries.
As for the rest of the world, an expert relied on by the New York Times for gun violence statistics told us there is no way to know how many school shootings -- using the definition Greenfield relies on -- have occurred outside of the United States over the past 20 years.
PolitiFact Wisconsin items as noted