"The full article details 12 different mass shootings, and looks at "whether proposals might have made a difference in how the guns were obtained, or whether existing laws worked as intended."

Yes, It's True That Gun Laws, Actual or Proposed, Would Not Have Stopped Recent Mass Shootings - Hit & Run : Reason.com: "Not so much news to readers of Reason as we alas have occasion to remind you of this anytime a gun murder makes big national news, but presidential candidate and Florida Senator Marco Rubio apparently shocked some people the other week when he declared on CBS's This Morning program that "None of the major shootings that have occurred in this country over the last few months or years that have outraged us, would gun laws have prevented them.” 

This seemed fishy to people who don't pay attention, for some reason, so the Washington Post's fact-checker was asked to investigate.  The full article details 12 different mass shootings, and looks at "whether proposals might have made a difference in how the guns were obtained, or whether existing laws worked as intended." Despite the suspicion that sometimes media fact checkers have agendas of their own, the truth is too obvious in this case. The Post gives Rubio's statement "a rare Geppetto checkmark" as their article demonstrates how neither existing nor proposed gun laws would have prevented them.

...other gun-related fact-checking news from the Post, they also cut through the prevarications from Sen. Dianne Feinstein (D-Calif.) and others about how their proposed bill killed by Republican in the Senate to pre-emptively deny Second Amendment rights to a class of citizens they like to refer to as terror suspects did not just apply to people on the more limited "no fly" list. Rather it would have entrapped everyone tarred with the incredibly broadbrush and secretive 800,000 people "terror watchlist" of unverified suspicion, with the no-fly list less than 10 percent that big. It was not, despite what you might hear, as simple as "if you are too dangerous to fly, you are too dangerous to buy a gun." "

Marco Rubio’s claim that no recent mass shootings would have been prevented by gun laws - The Washington Post: "This is certainly a depressing chronicle of death and tragedy. But Rubio’s statement stands up to scrutiny — at least for the recent past, as he framed it. Notably, three of the mass shootings took place in California, which already has strong gun laws including a ban on certain weapons and high-capacity magazines...  It is possible that some gun-control proposals, such as a ban on large-capacity magazines, would reduce the number of dead in a future shooting, though the evidence for that is heavily disputed. But Rubio was speaking in the past, about specific incidents. He earns a rare Geppetto Checkmark."