
The Atlantic Magazine (here) has looked at all the mass shootings since 2003 to determine whether the current bill being debated in the Senate today would have stopped any of the mass shootings. Result: one might have been stopped or 7.9% of the victims.
[O]f the 30 incidents since 2003 that we looked at, the Senate deal would quite possibly only have stopped one.
- Douglas Williams, Meridian, Mississippi, July 2003. Williams killed six people at a Lockheed Martin plant with a shotgun that he purchased from a private dealer — a sale that would, under the new Senate compromise, mandate a background check. Williams took anti-depressant medication following a 1989 divorce, which, in some circumstances, can prevent gun ownership.
“We decided to look at what recent history actually shows. Largely relying on Mother Jones‘s guide to mass shootings, we identified 30 incidents over the past decade, stretching back to July 2003.”

And they report that more than half the shooters underwent and passed background checks. How many people possibly, maybe, perhaps would have been saved 38, or 7.9% of all the deaths.