The problem is, what exactly counts as a "mass shooting"? Obviously multiple people being shot and killed is the main criteria. But we clearly wouldn't count the Battle of Gettysburg -- that's a military action. But the Fort Hood rampage is being included on lists of mass shootings, so apparently military personnel count if they aren't engaged in combat operations, which muddies the waters quite a bit.
But there's another sticking point that's far more serious. If we were to define a mass shooting as simply one person with a gun going around killing people in a single spree, things would be clear cut. But San Bernadino and Columbine are considered mass shootings, and those involved two gunmen. If two, then why not three? And if three why not a dozen? Or a hundred?
And that's where the assertion that this was the worst mass shooting in US history falls apart. If we were to limit the claim to the modern era (everything after World War II, let's say), it'd be a defensible claim, but once you include the 19th Century, it's a different matter entirely.
Just consider the Mountain Meadows Massacre where a group of Mormon settlers fell upon a wagon train passing through their territory and murdered more than a hundred men, women and children. The Mormons were organized in a paramilitary group called the Nauvoo Legion, but their actions at Mountain Meadows were much more akin to a terrorist group. We might disqualify Mountain Meadows as a mass shooting on the grounds that terrorism is something else, but that would also exclude San Bernadino, Fort Hood, probably Orlando, and, though not in the US, the Charlie Hebdo massacre as well.
But even if we do set aside Mountain Meadows, it's hardly unique in 19th Century history. The main thing that makes it noteworthy is that it was a case of white settlers killing other white settlers. More commonly whites were massacring the natives (and occasionally vice versa). Wikipedia has an extensive list. Just in the 1850s (the same decade as Mountain Meadows), we have:
Miners killed 300 Wintu Indians near Old Shasta, California and burned down their tribal council meeting house.
Californian settlers attacked and burned the Tolowa village of Howonquet, massacring 70 people.And yet the media doesn't count these as mass shootings. Most reporters are likely ignorant of such events, but there are some who know history and simply don't register Indian massacres as the same class of thing as the Orlando nightclub attack.
White settlers launched an attack on a Tolowa village near Lake Earl in California, killing between 65 and 150 Indians at dawn.
White settlers massacred 70 Achomawi Indians (10 men and 60 women and children) in their village on Pit River in California.
But they are. They are the exact same thing. They are Americans committing mass murder against vulnerable minorities for no other reason than somebody doesn't like the existence of that minority. Modern technology has made it so one guy can accomplish what it used to take dozens to do, but that doesn't change the nature of what's happening. To make the Orlando shootings out to be the worst thing that has ever happened is to whitewash a long and nasty history of oppression. To blame the attacks on the shooter's religion is to ignore the fact that he grew up in the United States where he was exposed to virulent homophobic rhetoric as a matter of course -- just because he's Muslim doesn't mean he's not affected by the Rick Santorums and Roy Moores of the world. Accepting extremist Christianity as normal makes other extremist creeds look equally reasonable. And in doing so, we allow the mindset behind Mountain Meadows and the Indian massacres to spread in the modern world. We need to face up to our history in order to put modern events in their proper context and understand how what's wrong with our world today is the product of our past.
And that means that instead of hyping the Orlando massacre like it's a blockbuster movie that's shattered box office records, as so many reporters seem intent upon doing, we need to see it as the latest in a centuries' long string of attacks against minorities, and admit that this is a recurrent problem in our culture that needs to be addressed.