So now they're claiming "more than" 150 mass shooting in the United States this year.
That was a concerted effort by a guy named Brock Weller, who runs a particularly toxic
anti-gun forum on Reddit (serious language at that link, folks). He started an archive known as the mass shooting tracker. He has people scour news reports for shootings with multiple injuries (not fatalities), which he classifies as "mass shootings." In some cases, the "shootings" were reports of shots fired with no injuries. In other cases, they were "shootings" done with pellet guns, negligent discharges by school security officers...the list goes on.
After Sandy Hook, he gave the media
exactly what they wanted, which was the claim we've had a mass shooting
every day in this country since then. Never mind the corrupt data or the fact he moved the very goal posts for how we define "mass shooting." He was able to back a spurious narrative with numbers, and that's all sciency, right?
Despite being roundly refuted, to the point
Mother Jones advised people not to trust it, the shooting tracker would be a valuable rhetorical device for politicians up to and including the President. It's easy because it makes for a good soundbite, and most people won't actually check the numbers.