To quote from memory another about our ever hardening position on decades of bad faith attacks on firearms ownership:
"How many children are we willing to let die?"
"Every single one of them."
Let me assure you we're very bit as callous as those who think that "a woman found dead in an alley, raped and strangled with her own pantyhose is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound."
You're just not going to disarm us, heck, we're winning at the state level more and more, as mentioned in another comment Idaho and WV are going Constitutional Carry, and they're not the only states where that's in play this legislative year.
Why? Well, we think these bad outcomes are more than outweighed by the good ones, like the ~2.5 million uses of guns in self-defense per year. Even in terms of raw numbers of dead and injured our approach probably wins, and we think the value of having this defense for the lives of our families and ourselves is of inestimable value.
At the extreme, we look at longer time frames, including the 20th Century, where substantially more than 100 million previously disarmed people were murdered by their own governments, or conquerors like in the bloodlands between Germany and the USSR.
As for your specifics:
There were 372 mass shootings in the US in 2015, killing 475 people and wounding 1,870
That "study", if you can call it that, is grossly fraudulent, you know you're in trouble when anti-gun Mother Jones calls you out: http://www.motherjones.com/politics/2015/12/no-there-were-no...
Ditto the "school shootings", that redefines the term to include gang sorts of crime.
As for how many killed and injured, we're an historically violent people, and an obsession on one particular instrument we use to express that violence is misplaced.